diff -r b40524b74f77 -r d6cb7e625d75 src/HOL/Library/Nat_Infinity.thy
--- a/src/HOL/Library/Nat_Infinity.thy	Fri Aug 20 17:46:55 2010 +0200
+++ b/src/HOL/Library/Nat_Infinity.thy	Fri Aug 20 17:46:56 2010 +0200
@@ -227,8 +227,10 @@
   apply (simp add: plus_1_iSuc iSuc_Fin)
   done
 
-instance inat :: semiring_char_0
-  by default (simp add: of_nat_eq_Fin)
+instance inat :: semiring_char_0 proof
+  have "inj Fin" by (rule injI) simp
+  then show "inj (\<lambda>n. of_nat n :: inat)" by (simp add: of_nat_eq_Fin)
+qed
 
 
 subsection {* Ordering *}