--- a/mercurial/configitems.py	Wed Feb 19 13:39:00 2020 +0530
+++ b/mercurial/configitems.py	Wed Feb 19 17:30:04 2020 +0100
@@ -759,7 +759,7 @@
 coreconfigitem(
     b'format',
     b'revlog-compression',
-    default=b'zlib',
+    default=lambda: [b'zlib'],
     alias=[(b'experimental', b'format.compression')],
 )
 coreconfigitem(