ctype_utf8.result, ctype_utf8.test:

  Adding test case.
item_func.cc:
  Bug#13751 find_in_set: Illegal mix of collations.
  Character set conversion was forgotten in find_in_set.

mysql-test/r/ctype_utf8.result

@@ -1056,3 +1056,8 @@ hex(a)
 5B
 E880BD
 drop table t1;
+set names 'latin1';
+create table t1 (a varchar(255)) default charset=utf8;
+select * from t1 where find_in_set('-1', a);
+a
+drop table t1;

mysql-test/t/ctype_utf8.test

@@ -865,4 +865,12 @@ insert into t1 values (_utf8 0x5b);
 select hex(a) from t1;
 drop table t1;
 
+#
+# Bug#13751 find_in_set: Illegal mix of collations
+#
+set names 'latin1';
+create table t1 (a varchar(255)) default charset=utf8;
+select * from t1 where find_in_set('-1', a);
+drop table t1;
+
 # End of 4.1 tests

sql/item_func.cc

@@ -1454,7 +1454,7 @@ void Item_func_find_in_set::fix_length_and_dec()
       }
     }
   }
-  agg_arg_collations_for_comparison(cmp_collation, args, 2);
+  agg_arg_charsets(cmp_collation, args, 2, MY_COLL_CMP_CONV);
 }
 
 static const char separator=',';