diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -48,7 +48,7 @@
     docket.data_unused = data_unused
 
     filename = _rawdata_filepath(revlog, docket)
-    use_mmap = revlog.opener.options.get("exp-persistent-nodemap.mmap")
+    use_mmap = revlog.opener.options.get(b"exp-persistent-nodemap.mmap")
     try:
         with revlog.opener(filename) as fd:
             if use_mmap:
@@ -132,7 +132,7 @@
     can_incremental = util.safehasattr(revlog.index, "nodemap_data_incremental")
     ondisk_docket = revlog._nodemap_docket
     feed_data = util.safehasattr(revlog.index, "update_nodemap_data")
-    use_mmap = revlog.opener.options.get("exp-persistent-nodemap.mmap")
+    use_mmap = revlog.opener.options.get(b"exp-persistent-nodemap.mmap")
 
     data = None
     # first attemp an incremental update of the data