diff --git a/mps/code/mpmtypes.h b/mps/code/mpmtypes.h
index 6f5dc1b4924..f7da1f58d7c 100644
--- a/mps/code/mpmtypes.h
+++ b/mps/code/mpmtypes.h
@@ -427,7 +427,7 @@ enum {
 enum {
   MessageTypeFINALIZATION,  /* MPS_MESSAGE_TYPE_FINALIZATION */
   MessageTypeGC,  /* MPS_MESSAGE_TYPE_GC */
-  MessageTypeGCSTART,
+  MessageTypeGCSTART,  /* MPS_MESSAGE_TYPE_GC_START */
   MessageTypeLIMIT /* not a message type, the limit of the enum. */
 };