diff --git a/lib/galaxy/datatypes/registry.py b/lib/galaxy/datatypes/registry.py
index 7a1629a0579..dd46b56626d 100644
--- a/lib/galaxy/datatypes/registry.py
+++ b/lib/galaxy/datatypes/registry.py
@@ -75,7 +75,7 @@ class Registry(object):
         self.xml_filename = None
         self._edam_formats_mapping = None
         self._edam_data_mapping = None
-        self._converters_by_datatype = None
+        self._converters_by_datatype = {}
         # Build sites
         self.build_sites = {}
         self.display_sites = {}
@@ -798,7 +798,7 @@ class Registry(object):
 
     def get_converters_by_datatype(self, ext):
         """Returns available converters by source type"""
-        if not self._converters_by_datatype:
+        if ext not in self._converters_by_datatype:
             converters = odict()
             source_datatype = type(self.get_datatype_by_extension(ext))
             for ext2, converters_dict in self.datatype_converters.items():
@@ -808,8 +808,8 @@ class Registry(object):
             # Ensure ext-level converters are present
             if ext in self.datatype_converters.keys():
                 converters.update(self.datatype_converters[ext])
-            self._converters_by_datatype = converters
-        return self._converters_by_datatype
+            self._converters_by_datatype[ext] = converters
+        return self._converters_by_datatype[ext]
 
     def get_converter_by_target_type(self, source_ext, target_ext):
         """Returns a converter based on source and target datatypes"""