diff --git a/introspection/org.freedesktop.ModemManager1.Sms.xml b/introspection/org.freedesktop.ModemManager1.Sms.xml
index 16e29d2f..586c2fd6 100644
--- a/introspection/org.freedesktop.ModemManager1.Sms.xml
+++ b/introspection/org.freedesktop.ModemManager1.Sms.xml
@@ -122,11 +122,12 @@
     <!--
         Class:
 
-        3GPP message class (0..3).
+        3GPP message class (-1..3).  -1 means class is not available or
+        is not used for this message, otherwise the 3GPP SMS message class.
 
-        Always 0 for 3GPP2/CDMA.
+        Always -1 for 3GPP2/CDMA.
     -->
-    <property name="Class" type="u" access="read" />
+    <property name="Class" type="i" access="read" />
 
     <!--
         DeliveryReportRequest:
diff --git a/libmm-glib/mm-sms-properties.c b/libmm-glib/mm-sms-properties.c
index 05d5fd16..a6947269 100644
--- a/libmm-glib/mm-sms-properties.c
+++ b/libmm-glib/mm-sms-properties.c
@@ -51,8 +51,7 @@ struct _MMSmsPropertiesPrivate {
     gchar *smsc;
     MMSmsValidityType validity_type;
     guint validity_relative;
-    gboolean class_set;
-    guint class;
+    gint class;
     gboolean delivery_report_request_set;
     gboolean delivery_report_request;
 };
@@ -319,17 +318,16 @@ mm_sms_properties_get_validity_relative (MMSmsProperties *self)
 /**
  * mm_sms_properties_set_class:
  * @self: A #MMSmsProperties.
- * @class: The message class.
+ * @class: The message class, or -1 for invalid/unset class.
  *
  * Sets the 3GPP message class of the SMS.
  */
 void
 mm_sms_properties_set_class (MMSmsProperties *self,
-                             guint class)
+                             gint class)
 {
     g_return_if_fail (MM_IS_SMS_PROPERTIES (self));
 
-    self->priv->class_set = TRUE;
     self->priv->class = class;
 }
 
@@ -339,12 +337,12 @@ mm_sms_properties_set_class (MMSmsProperties *self,
  *
  * Gets the 3GPP message class of the SMS.
  *
- * Returns: the message class.
+ * Returns: the message class, or -1 for invalid/unset class.
  */
-guint
+gint
 mm_sms_properties_get_class (MMSmsProperties *self)
 {
-    g_return_val_if_fail (MM_IS_SMS_PROPERTIES (self), 0);
+    g_return_val_if_fail (MM_IS_SMS_PROPERTIES (self), -1);
 
     return self->priv->class;
 }
@@ -435,11 +433,11 @@ mm_sms_properties_get_dictionary (MMSmsProperties *self)
                                PROPERTY_VALIDITY,
                                g_variant_new ("(uv)", MM_SMS_VALIDITY_TYPE_RELATIVE, g_variant_new_uint32 (self->priv->validity_relative)));
 
-    if (self->priv->class_set)
+    if (self->priv->class >= 0)
         g_variant_builder_add (&builder,
                                "{sv}",
                                PROPERTY_CLASS,
-                               g_variant_new_uint32 (self->priv->class));
+                               g_variant_new_int32 (self->priv->class));
 
     if (self->priv->delivery_report_request_set)
         g_variant_builder_add (&builder,
@@ -517,12 +515,14 @@ consume_string (MMSmsProperties *self,
 
         mm_sms_properties_set_validity_relative (self, n);
     } else if (g_str_equal (key, PROPERTY_CLASS)) {
-        GError *inner_error = NULL;
-        guint n;
+        gint n = 0;
 
-        n = parse_uint (value, &inner_error);
-        if (inner_error) {
-            g_propagate_error (error, inner_error);
+        if (!mm_get_int_from_str (value, &n)) {
+            g_set_error (error,
+                         MM_CORE_ERROR,
+                         MM_CORE_ERROR_INVALID_ARGS,
+                         "Invalid properties string, cannot parse '%s' as int",
+                         value);
             return FALSE;
         }
 
@@ -642,7 +642,7 @@ consume_variant (MMSmsProperties *properties,
     } else if (g_str_equal (key, PROPERTY_CLASS))
         mm_sms_properties_set_class (
             properties,
-            g_variant_get_uint32 (value));
+            g_variant_get_int32 (value));
     else if (g_str_equal (key, PROPERTY_DELIVERY_REPORT_REQUEST))
         mm_sms_properties_set_delivery_report_request (
             properties,
@@ -746,6 +746,7 @@ mm_sms_properties_init (MMSmsProperties *self)
                                               MM_TYPE_SMS_PROPERTIES,
                                               MMSmsPropertiesPrivate);
     self->priv->validity_type = MM_SMS_VALIDITY_TYPE_UNKNOWN;
+    self->priv->class = -1;
 }
 
 static void
diff --git a/libmm-glib/mm-sms-properties.h b/libmm-glib/mm-sms-properties.h
index 82c65595..b0c290a0 100644
--- a/libmm-glib/mm-sms-properties.h
+++ b/libmm-glib/mm-sms-properties.h
@@ -71,7 +71,7 @@ void mm_sms_properties_set_smsc                    (MMSmsProperties *self,
 void mm_sms_properties_set_validity_relative       (MMSmsProperties *self,
                                                     guint validity);
 void mm_sms_properties_set_class                   (MMSmsProperties *self,
-                                                    guint class);
+                                                    gint class);
 void mm_sms_properties_set_delivery_report_request (MMSmsProperties *self,
                                                     gboolean request);
 
@@ -84,7 +84,7 @@ const gchar  *mm_sms_properties_get_number                  (MMSmsProperties *se
 const gchar  *mm_sms_properties_get_smsc                    (MMSmsProperties *self);
 MMSmsValidityType mm_sms_properties_get_validity_type       (MMSmsProperties *self);
 guint         mm_sms_properties_get_validity_relative       (MMSmsProperties *self);
-guint         mm_sms_properties_get_class                   (MMSmsProperties *self);
+gint          mm_sms_properties_get_class                   (MMSmsProperties *self);
 gboolean      mm_sms_properties_get_delivery_report_request (MMSmsProperties *self);
 
 /*****************************************************************************/
diff --git a/libmm-glib/mm-sms.c b/libmm-glib/mm-sms.c
index a69b5dc7..82290218 100644
--- a/libmm-glib/mm-sms.c
+++ b/libmm-glib/mm-sms.c
@@ -445,12 +445,12 @@ mm_sms_get_validity_relative (MMSms *self)
  *
  * Gets the 3GPP message class of the SMS.
  *
- * Returns: the message class.
+ * Returns: the message class, or -1 for invalid/unset class.
  */
-guint
+gint
 mm_sms_get_class (MMSms *self)
 {
-    g_return_val_if_fail (MM_IS_SMS (self), 0);
+    g_return_val_if_fail (MM_IS_SMS (self), -1);
 
     return mm_gdbus_sms_get_class (MM_GDBUS_SMS (self));
 }
diff --git a/libmm-glib/mm-sms.h b/libmm-glib/mm-sms.h
index cb5b2b9d..def70c10 100644
--- a/libmm-glib/mm-sms.h
+++ b/libmm-glib/mm-sms.h
@@ -89,7 +89,7 @@ gchar        *mm_sms_dup_discharge_timestamp     (MMSms *self);
 MMSmsValidityType mm_sms_get_validity_type       (MMSms *self);
 guint             mm_sms_get_validity_relative   (MMSms *self);
 
-guint         mm_sms_get_class                   (MMSms *self);
+gint          mm_sms_get_class                   (MMSms *self);
 
 guint         mm_sms_get_message_reference       (MMSms *self);
 
diff --git a/src/mm-broadband-modem.c b/src/mm-broadband-modem.c
index 45d9027a..33e3246f 100644
--- a/src/mm-broadband-modem.c
+++ b/src/mm-broadband-modem.c
@@ -5828,7 +5828,7 @@ sms_text_part_list_ready (MMBroadbandModem *self,
         mm_sms_part_take_timestamp (part, timestamp);
         mm_sms_part_take_text (part, text);
         mm_sms_part_take_data (part, raw);
-        mm_sms_part_set_class (part, 0);
+        mm_sms_part_set_class (part, -1);
 
         mm_dbg ("Correctly parsed SMS list entry (%d)", idx);
         mm_iface_modem_messaging_take_part (MM_IFACE_MODEM_MESSAGING (self),
diff --git a/src/mm-sms-part.c b/src/mm-sms-part.c
index a5e8b33c..77fa3b82 100644
--- a/src/mm-sms-part.c
+++ b/src/mm-sms-part.c
@@ -325,7 +325,7 @@ struct _MMSmsPart {
     gchar *text;
     MMSmsEncoding encoding;
     GByteArray *data;
-    guint class;
+    gint  class;
     guint validity_relative;
     gboolean delivery_report_request;
     guint message_reference;
@@ -403,8 +403,8 @@ PART_GET_FUNC (const gchar *, text)
 PART_SET_TAKE_STR_FUNC (text)
 PART_GET_FUNC (MMSmsEncoding, encoding)
 PART_SET_FUNC (MMSmsEncoding, encoding)
-PART_GET_FUNC (guint, class)
-PART_SET_FUNC (guint, class)
+PART_GET_FUNC (gint,  class)
+PART_SET_FUNC (gint,  class)
 PART_GET_FUNC (guint, validity_relative)
 PART_SET_FUNC (guint, validity_relative)
 PART_GET_FUNC (gboolean, delivery_report_request)
@@ -461,6 +461,7 @@ mm_sms_part_new (guint index,
     sms_part->pdu_type = pdu_type;
     sms_part->encoding = MM_SMS_ENCODING_UNKNOWN;
     sms_part->delivery_state = MM_SMS_DELIVERY_STATE_UNKNOWN;
+    sms_part->class = -1;
 
     return sms_part;
 }
@@ -917,12 +918,7 @@ mm_sms_part_encode_address (const gchar *address,
 /**
  * mm_sms_part_get_submit_pdu:
  *
- * @number: the subscriber number to send this message to
- * @text: the body of this SMS
- * @smsc: if given, the SMSC address
- * @validity: minutes until the SMS should expire in the SMSC, or 0 for a
- *  suitable default
- * @class: unused
+ * @part: the SMS message part
  * @out_pdulen: on success, the size of the returned PDU in bytes
  * @out_msgstart: on success, the byte index in the returned PDU where the
  *  message starts (ie, skipping the SMSC length byte and address, if present)
@@ -1023,6 +1019,12 @@ mm_sms_part_get_submit_pdu (MMSmsPart *part,
     /* ----------- TP-DCS (1 byte) ----------- */
     pdu[offset] = 0x00;
 
+    if (part->class >= 0 && part->class <= 3) {
+        mm_dbg ("  using class %d...", part->class);
+        pdu[offset] |= SMS_DCS_CLASS_VALID;
+        pdu[offset] |= part->class;
+    }
+
     if (part->encoding == MM_SMS_ENCODING_UCS2) {
         mm_dbg ("  using UCS2 encoding...");
         pdu[offset] |= SMS_DCS_CODING_UCS2;
diff --git a/src/mm-sms-part.h b/src/mm-sms-part.h
index 8e7cdd92..d44ed5bc 100644
--- a/src/mm-sms-part.h
+++ b/src/mm-sms-part.h
@@ -96,9 +96,9 @@ MMSmsEncoding     mm_sms_part_get_encoding           (MMSmsPart *part);
 void              mm_sms_part_set_encoding           (MMSmsPart *part,
                                                       MMSmsEncoding encoding);
 
-guint             mm_sms_part_get_class              (MMSmsPart *part);
+gint              mm_sms_part_get_class              (MMSmsPart *part);
 void              mm_sms_part_set_class              (MMSmsPart *part,
-                                                      guint class);
+                                                      gint class);
 
 guint             mm_sms_part_get_validity_relative  (MMSmsPart *part);
 void              mm_sms_part_set_validity_relative  (MMSmsPart *part,
diff --git a/src/tests/test-sms-part.c b/src/tests/test-sms-part.c
index 6a3f7183..d1f75886 100644
--- a/src/tests/test-sms-part.c
+++ b/src/tests/test-sms-part.c
@@ -512,7 +512,7 @@ common_test_create_pdu (const gchar *smsc,
                         const gchar *number,
                         const gchar *text,
                         guint validity,
-                        guint class,
+                        gint  class,
                         const guint8 *expected,
                         gsize expected_size,
                         guint expected_msgstart)
@@ -537,7 +537,7 @@ common_test_create_pdu (const gchar *smsc,
     }
     if (validity > 0)
         mm_sms_part_set_validity_relative (part, validity);
-    if (class > 0)
+    if (class >= 0)
         mm_sms_part_set_class (part, class);
 
     pdu = mm_sms_part_get_submit_pdu (part,
@@ -576,7 +576,7 @@ test_create_pdu_ucs2_with_smsc (void)
                             number,
                             text,
                             5, /* validity */
-                            0, /* class */
+                            -1, /* class */
                             expected,
                             sizeof (expected),
                             8); /* expected_msgstart */
@@ -601,7 +601,7 @@ test_create_pdu_ucs2_no_smsc (void)
                             number,
                             text,
                             5, /* validity */
-                            0, /* class */
+                            -1, /* class */
                             expected,
                             sizeof (expected),
                             1); /* expected_msgstart */
@@ -626,7 +626,7 @@ test_create_pdu_gsm_with_smsc (void)
                             number,
                             text,
                             5, /* validity */
-                            0, /* class */
+                            -1, /* class */
                             expected,
                             sizeof (expected),
                             8); /* expected_msgstart */
@@ -650,7 +650,7 @@ test_create_pdu_gsm_no_smsc (void)
                             number,
                             text,
                             5, /* validity */
-                            0, /* class */
+                            -1, /* class */
                             expected,
                             sizeof (expected),
                             1); /* expected_msgstart */
@@ -678,7 +678,7 @@ test_create_pdu_gsm_3 (void)
                             number,
                             text,
                             5, /* validity */
-                            0, /* class */
+                            -1, /* class */
                             expected,
                             sizeof (expected),
                             1); /* expected_msgstart */
@@ -699,7 +699,7 @@ test_create_pdu_gsm_no_validity (void)
                             number,
                             text,
                             0, /* validity */
-                            0, /* class */
+                            -1, /* class */
                             expected,
                             sizeof (expected),
                             1); /* expected_msgstart */