diff --git a/src/proto/alpha/docs/language.md b/src/proto/alpha/docs/language.md
index b75af370e..a28a44ddb 100644
--- a/src/proto/alpha/docs/language.md
+++ b/src/proto/alpha/docs/language.md
@@ -755,7 +755,7 @@ constants as is, concatenate them and use them as keys.
    * `MEM`:
      Check for the presence of an element in a set.
 
-        :: 'key : set 'elt : 'S   ->  bool : 'S
+        :: 'elt : set 'elt : 'S   ->  bool : 'S
 
    * `UPDATE`:
      Inserts or removes an element in a set, replacing a previous value.