--- a/mercurial/revlogutils/nodemap.py	Wed Dec 09 18:51:52 2020 -0800
+++ b/mercurial/revlogutils/nodemap.py	Mon Dec 07 18:06:53 2020 +0100
@@ -58,7 +58,9 @@
             else:
                 data = fd.read(data_length)
     except OSError as e:
-        if e.errno != errno.ENOENT:
+        if e.errno == errno.ENOENT:
+            return None
+        else:
             raise
     if len(data) < data_length:
         return None