diff --git a/mpegize.py b/mpegize.py
index 3a487d5..056e033 100755
--- a/mpegize.py
+++ b/mpegize.py
@@ -445,7 +445,7 @@ class Mpegize:
                 
                 print("Message: proc")
                 sys.stdout.flush()
-                gst_convert(m.base, argv[3], argv[4]=="dng")
+                gst_convert(m.base, argv[3], ext=="dng")
                 m.cleanup()
                 return
             if mode == "convert" or mode == "stop":
@@ -459,7 +459,7 @@ class Mpegize:
                 print("Phase 2: mpegize -- ", argv[3], file=sys.stderr)
                 print("Message: enc")
                 sys.stdout.flush()
-                gst_convert(m.base, argv[3], argv[4]=="dng")
+                gst_convert(m.base, argv[3], ext=="dng")
                 m.cleanup()
                 return
             if mode == "gaps":