diff -r 7466b2a3905a -r 42523fbf643b src/HOL/String.thy
--- a/src/HOL/String.thy	Mon Sep 13 13:30:39 2021 +0200
+++ b/src/HOL/String.thy	Mon Sep 13 14:18:24 2021 +0000
@@ -121,7 +121,7 @@
 
 lemma char_of_nat [simp]:
   \<open>char_of (of_nat n) = char_of n\<close>
-  by (simp add: char_of_def String.char_of_def drop_bit_of_nat bit_simps)
+  by (simp add: char_of_def String.char_of_def drop_bit_of_nat bit_simps possible_bit_def)
 
 end