diff --git a/wptserve/stash.py b/wptserve/stash.py
index 410ddf0643617a..56476176e82633 100644
--- a/wptserve/stash.py
+++ b/wptserve/stash.py
@@ -119,7 +119,7 @@ def put(self, key, value, path=None):
         if internal_key in self.data:
             raise StashError("Tried to overwrite existing shared stash value "
                              "for key %s (old value was %s, new value is %s)" %
-                             (internal_key, self[str(internal_key)], value))
+                             (internal_key, self.data[str(internal_key)], value))
         else:
             self.data[internal_key] = value