diff --git a/lib/linguist/languages.yml b/lib/linguist/languages.yml
index e5b1a6bf..4aeda855 100644
--- a/lib/linguist/languages.yml
+++ b/lib/linguist/languages.yml
@@ -787,7 +787,7 @@ M:
   lexer: Common Lisp
   aliases:
   - mumps
-  primary_extension: .m
+  primary_extension: .mumps
 
 Makefile:
   aliases: