diff --git a/expression/builtin_compare_vec.go b/expression/builtin_compare_vec.go
index c1c8cc18c0c48..439143706bf45 100644
--- a/expression/builtin_compare_vec.go
+++ b/expression/builtin_compare_vec.go
@@ -254,6 +254,7 @@ func (b *builtinLeastStringSig) vecEvalString(input *chunk.Chunk, result *chunk.
 	src := result
 	arg := buf1
 	dst := buf2
+	dst.ReserveString(n)
 	for j := 1; j < len(b.args); j++ {
 		if err := b.args[j].VecEvalString(b.ctx, input, arg); err != nil {
 			return err
@@ -793,6 +794,7 @@ func (b *builtinGreatestStringSig) vecEvalString(input *chunk.Chunk, result *chu
 	src := result
 	arg := buf1
 	dst := buf2
+	dst.ReserveString(n)
 	for j := 1; j < len(b.args); j++ {
 		if err := b.args[j].VecEvalString(b.ctx, input, arg); err != nil {
 			return err
diff --git a/util/chunk/column.go b/util/chunk/column.go
index 4089389872318..fd51fb2bb056b 100644
--- a/util/chunk/column.go
+++ b/util/chunk/column.go
@@ -703,7 +703,7 @@ func (c *Column) MergeNulls(cols ...*Column) {
 	}
 	for _, col := range cols {
 		if c.length != col.length {
-			panic("should ensure all columns have the same length")
+			panic(fmt.Sprintf("should ensure all columns have the same length, expect %v, but got %v", c.length, col.length))
 		}
 	}
 	for _, col := range cols {