diff -r cccbfa567117 -r b69e4da2604b src/HOL/Library/Multiset.thy
--- a/src/HOL/Library/Multiset.thy	Fri Jun 06 18:36:29 2025 +0100
+++ b/src/HOL/Library/Multiset.thy	Mon Jun 09 22:14:38 2025 +0200
@@ -4318,7 +4318,7 @@
   by simp
 
 lemma [code]: "Multiset.is_empty (mset xs) \<longleftrightarrow> List.null xs"
-  by (simp add: Multiset.is_empty_def List.null_def)
+  by (simp add: Multiset.is_empty_def)
 
 lemma union_code [code]: "mset xs + mset ys = mset (xs @ ys)"
   by simp