user_var.result, user_var.test:

  My previous change that "set @A=NULL" doesn't change charset
  fixed 'Bug #6321' as well. Prove with a new test that
  FIELD(<uservariable content NULL>, ...) now works fine too.

mysql-test/r/user_var.result

@@ -195,3 +195,7 @@ SET @`v`:=_ucs2 0x006100620063 COLLATE ucs2_general_ci;
 SET TIMESTAMP=10000;
 insert into t2 values (@v);
 drop table t1, t2;
+set @var= NULL ;
+select FIELD( @var,'1it','Hit') as my_column;
+my_column
+0

mysql-test/t/user_var.test

@@ -120,3 +120,9 @@ show binlog events from 79;
 drop table t1, t2;
 
 
+#
+# Bug #6321 strange error:
+#   string function FIELD(<uservariable content NULL>, ...)
+#
+set @var= NULL ;
+select FIELD( @var,'1it','Hit') as my_column;