diff --git a/mysql-test/r/key.result b/mysql-test/r/key.result
index e3b341fcaf8..e74bda23da9 100644
--- a/mysql-test/r/key.result
+++ b/mysql-test/r/key.result
@@ -305,3 +305,5 @@ check table t1;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 drop table t1;
+create table t1 (c char(10), index (c(0)));
+ERROR HY000: Key part 'c' length cannot be 0
diff --git a/mysql-test/t/key.test b/mysql-test/t/key.test
index 662baa5ea9d..4be34cac30e 100644
--- a/mysql-test/t/key.test
+++ b/mysql-test/t/key.test
@@ -285,3 +285,14 @@ check table t1;
 
 drop table t1;
 
+#
+# Bug 6166: index prefix length of 0 not rejected
+#
+# this test should fail in 5.0
+# to fix it, remove #ifdef in 
+# file sql_yacc.yy(key_part)
+# create dedicated error code for this and
+# and change my_printf_error() to my_error
+
+--error 1105
+create table t1 (c char(10), index (c(0)));
diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index 426f6d4d057..8aad067944a 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -1817,7 +1817,19 @@ key_list:
 
 key_part:
 	ident			{ $$=new key_part_spec($1.str); }
-	| ident '(' NUM ')'	{ $$=new key_part_spec($1.str,(uint) atoi($3.str)); };
+	| ident '(' NUM ')'	
+        {
+          int key_part_len= atoi($3.str);
+#ifdef MYSQL_VERSION_ID < 50000
+          if (!key_part_len)
+          {
+            my_printf_error(ER_UNKNOWN_ERROR,
+                            "Key part '%s' length cannot be 0",
+                            MYF(0), $1.str);
+          }
+#endif
+          $$=new key_part_spec($1.str,(uint) key_part_len);
+        };
 
 opt_ident:
 	/* empty */	{ $$=(char*) 0; }	/* Defaultlength */