--- a/mercurial/tags.py	Fri Apr 26 17:39:07 2019 +0200
+++ b/mercurial/tags.py	Mon Mar 11 01:09:38 2019 +0100
@@ -691,6 +691,9 @@
         If an .hgtags does not exist at the specified revision, nullid is
         returned.
         """
+        if node == nullid:
+            return nullid
+
         ctx = self._repo[node]
         rev = ctx.rev()