diff --git a/mercurial/unionrepo.py b/mercurial/unionrepo.py
--- a/mercurial/unionrepo.py
+++ b/mercurial/unionrepo.py
@@ -116,6 +116,9 @@ class unionrevlog(revlog.revlog):
             # already cached
         return text
 
+    def rawdata(self, nodeorrev, _df=None):
+        return self.revision(nodeorrev, _df=_df, raw=True)
+
     def baserevision(self, nodeorrev):
         # Revlog subclasses may override 'revision' method to modify format of
         # content retrieved from revlog. To use unionrevlog with such class one