diff --git a/atrcopy/dos33.py b/atrcopy/dos33.py
index d7c24fc..36e2838 100644
--- a/atrcopy/dos33.py
+++ b/atrcopy/dos33.py
@@ -718,3 +718,4 @@ class ProdosDiskImage(DiskImageBase):
                     #raise InvalidDiskImage("No ProDOS header info found")
                     pass
             raise UnsupportedDiskImage("ProDOS format found but not supported")
+        raise InvalidDiskImage("Not ProDOS format")