diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index b1994ff31638..f5f7e907d277 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -113,6 +113,7 @@ checkConfigError 'A definition for option .intStrings\.mergeError. is not of typ
 checkConfigError 'A definition for option .intStrings\.badTagError. is not of type .attribute-tagged union' config.intStrings.badTagError ./types-attrTag.nix
 checkConfigError 'A definition for option .intStrings\.badTagTypeError\.left. is not of type .signed integer.' config.intStrings.badTagTypeError.left ./types-attrTag.nix
 checkConfigError 'A definition for option .nested\.right\.left. is not of type .signed integer.' config.nested.right.left ./types-attrTag.nix
+checkConfigError 'In attrTag/attrTagWith, each tag value must be an option, but tag int was a bare type, not wrapped in mkOption.' config.opt.int ./types-attrTag-wrong-decl.nix
 
 # types.pathInStore
 checkConfigOutput '".*/store/0lz9p8xhf89kb1c1kk6jxrzskaiygnlh-bash-5.2-p15.drv"' config.pathInStore.ok1 ./types.nix
diff --git a/lib/tests/modules/types-attrTag-wrong-decl.nix b/lib/tests/modules/types-attrTag-wrong-decl.nix
new file mode 100644
index 000000000000..d03370bc10da
--- /dev/null
+++ b/lib/tests/modules/types-attrTag-wrong-decl.nix
@@ -0,0 +1,14 @@
+{ lib, ... }:
+let
+  inherit (lib) types mkOption;
+in
+{
+  options = {
+    opt = mkOption {
+      type = types.attrTag {
+        int = types.int;
+      };
+      default = { int = 1; };
+    };
+  };
+}
diff --git a/lib/types.nix b/lib/types.nix
index 7fe7b5d92eac..a77a8ef11244 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -15,6 +15,7 @@ let
     isList
     isString
     isStorePath
+    throwIf
     toDerivation
     toList
     ;
@@ -627,7 +628,13 @@ rec {
           mapAttrs
             (n: opt:
               builtins.addErrorContext "while checking that attrTag tag ${lib.strings.escapeNixIdentifier n} is an option with a type${inAttrPosSuffix args.tags n}" (
-                assert opt._type == "option";
+                throwIf (opt._type or null != "option")
+                  "In attrTag/attrTagWith, each tag value must be an option, but tag ${lib.strings.escapeNixIdentifier n} ${
+                    if opt?_type then
+                      if opt._type == "option-type"
+                      then "was a bare type, not wrapped in mkOption."
+                      else "was of type ${lib.strings.escapeNixString opt._type}."
+                    else "was not."}"
                 opt // {
                   declarations = opt.declarations or (
                     let pos = builtins.unsafeGetAttrPos n args.tags;