diff --git a/pkg/ddl/index.go b/pkg/ddl/index.go
index e232e24621ea8..4a068d8156045 100644
--- a/pkg/ddl/index.go
+++ b/pkg/ddl/index.go
@@ -217,7 +217,7 @@ func indexColumnsLen(cols []*model.ColumnInfo, idxCols []*model.IndexColumn, col
 
 // checkIndexColumn will be run for all non-columnar indexes.
 func checkIndexColumn(col *model.ColumnInfo, indexColumnLen int, suppressTooLongKeyErr bool) error {
-	if col.GetFlen() == 0 && (types.IsTypeChar(col.FieldType.GetType()) || types.IsTypeVarchar(col.FieldType.GetType())) {
+	if col.FieldType.GetType() == mysql.TypeNull || (col.GetFlen() == 0 && (types.IsTypeChar(col.FieldType.GetType()) || types.IsTypeVarchar(col.FieldType.GetType()))) {
 		if col.Hidden {
 			return errors.Trace(dbterror.ErrWrongKeyColumnFunctionalIndex.GenWithStackByArgs(col.GeneratedExprString))
 		}
diff --git a/tests/integrationtest/r/ddl/db_integration.result b/tests/integrationtest/r/ddl/db_integration.result
index 18c25997e3590..0f876949e1f6c 100644
--- a/tests/integrationtest/r/ddl/db_integration.result
+++ b/tests/integrationtest/r/ddl/db_integration.result
@@ -1239,6 +1239,8 @@ Error 3800 (HY000): Expression of expression index 'k' cannot refer to a row val
 create table t (j json, key k ((j+1),(j+1)));
 create table t1 (col1 int, index ((concat(''))));
 Error 3761 (HY000): The used storage engine cannot index the expression 'concat(_utf8mb4'')'
+create table t1 (col1 int, index ((null)));
+Error 3761 (HY000): The used storage engine cannot index the expression 'null'
 CREATE TABLE t1 (col1 INT, PRIMARY KEY ((ABS(col1))) NONCLUSTERED);
 Error 3756 (HY000): The primary key cannot be an expression index
 drop table if exists t;
diff --git a/tests/integrationtest/t/ddl/db_integration.test b/tests/integrationtest/t/ddl/db_integration.test
index 0857c6ddec31b..a2d61e739fd4f 100644
--- a/tests/integrationtest/t/ddl/db_integration.test
+++ b/tests/integrationtest/t/ddl/db_integration.test
@@ -1052,6 +1052,8 @@ create table t (j json, key k (((j,j))));
 create table t (j json, key k ((j+1),(j+1)));
 -- error 3761
 create table t1 (col1 int, index ((concat(''))));
+-- error 3761
+create table t1 (col1 int, index ((null)));
 -- error 3756
 CREATE TABLE t1 (col1 INT, PRIMARY KEY ((ABS(col1))) NONCLUSTERED);