diff --git a/lib/galaxy/datatypes/display_applications/parameters.py b/lib/galaxy/datatypes/display_applications/parameters.py
index d6b831396ec..f6b0b2bdba1 100644
--- a/lib/galaxy/datatypes/display_applications/parameters.py
+++ b/lib/galaxy/datatypes/display_applications/parameters.py
@@ -83,7 +83,7 @@ class DisplayApplicationDataParameter(DisplayApplicationParameter):
             data = data.value
         if self.metadata:
             rval = getattr(data.metadata, self.metadata, None)
-            assert rval, 'Unknown metadata name ({}) provided for dataset type ({}).'.format(self.metadata, data.datatype.__class__.name)
+            assert rval, 'Unknown metadata name "{}" provided for datatype "{}".'.format(self.metadata, data.ext)
             return Bunch(file_name=rval.file_name, state=data.state, states=data.states, extension='data')
         elif self.extensions and (self.force_conversion or not isinstance(data.datatype, self.formats)):
             for ext in self.extensions: