diff --git a/src/common/KeyMap.cxx b/src/common/KeyMap.cxx
index 559a7128a..7fa9b0ab9 100644
--- a/src/common/KeyMap.cxx
+++ b/src/common/KeyMap.cxx
@@ -300,7 +300,8 @@ json KeyMap::convertLegacyMapping(string list)
     mapping["event"] = Event::Type(event);
     mapping["key"] = StellaKey(key);
 
-    if(StellaMod(mod) != StellaMod::KBDM_NONE) mapping["mod"] = StellaMod(mod);
+    if(StellaMod(mod) != StellaMod::KBDM_NONE)
+      mapping["mod"] = serializeModkeyMask(StellaMod(mod));
 
     convertedMapping.push_back(mapping);
   }