diff --git a/lms/lib/xblock/runtime.py b/lms/lib/xblock/runtime.py
index f02657c511..6195048190 100644
--- a/lms/lib/xblock/runtime.py
+++ b/lms/lib/xblock/runtime.py
@@ -196,3 +196,11 @@ class LmsModuleSystem(LmsHandlerUrls, ModuleSystem):  # pylint: disable=abstract
         )
         services['fs'] = xblock.reference.plugins.FSService()
         super(LmsModuleSystem, self).__init__(**kwargs)
+
+    # backward compatibility fix for callers not knowing this is a ModuleSystem v DescriptorSystem
+    @property
+    def resources_fs(self):
+        """
+        Return what would be the resources_fs on a DescriptorSystem
+        """
+        return getattr(self, 'filestore', None)