diff --git a/weakmap.c b/weakmap.c
index f8a6ae3747..2ebf7d204f 100644
--- a/weakmap.c
+++ b/weakmap.c
@@ -859,7 +859,7 @@ wkmap_aset(VALUE self, VALUE key, VALUE val)
     TypedData_Get_Struct(self, struct weakkeymap, &weakkeymap_type, w);
 
     if (!FL_ABLE(key) || SYMBOL_P(key) || RB_BIGNUM_TYPE_P(key) || RB_TYPE_P(key, T_FLOAT)) {
-        rb_raise(rb_eArgError, "WeakKeyMap must be garbage collectable");
+        rb_raise(rb_eArgError, "WeakKeyMap keys must be garbage collectable");
         UNREACHABLE_RETURN(Qnil);
     }