diff --git a/executor/aggfuncs/func_group_concat.go b/executor/aggfuncs/func_group_concat.go
index 40ed32bce0093..68d4584f3d543 100644
--- a/executor/aggfuncs/func_group_concat.go
+++ b/executor/aggfuncs/func_group_concat.go
@@ -64,7 +64,8 @@ func (e *baseGroupConcat4String) truncatePartialResultIfNeed(sctx sessionctx.Con
 }
 
 type basePartialResult4GroupConcat struct {
-	buffer *bytes.Buffer
+	valsBuf *bytes.Buffer
+	buffer  *bytes.Buffer
 }
 
 type partialResult4GroupConcat struct {
@@ -76,7 +77,9 @@ type groupConcat struct {
 }
 
 func (e *groupConcat) AllocPartialResult() PartialResult {
-	return PartialResult(new(partialResult4GroupConcat))
+	p := new(partialResult4GroupConcat)
+	p.valsBuf = &bytes.Buffer{}
+	return PartialResult(p)
 }
 
 func (e *groupConcat) ResetPartialResult(pr PartialResult) {
@@ -86,12 +89,9 @@ func (e *groupConcat) ResetPartialResult(pr PartialResult) {
 
 func (e *groupConcat) UpdatePartialResult(sctx sessionctx.Context, rowsInGroup []chunk.Row, pr PartialResult) (err error) {
 	p := (*partialResult4GroupConcat)(pr)
-	v, isNull, preLen := "", false, 0
+	v, isNull := "", false
 	for _, row := range rowsInGroup {
-		if p.buffer != nil && p.buffer.Len() != 0 {
-			preLen = p.buffer.Len()
-			p.buffer.WriteString(e.sep)
-		}
+		p.valsBuf.Reset()
 		for _, arg := range e.args {
 			v, isNull, err = arg.EvalString(sctx, row)
 			if err != nil {
@@ -100,17 +100,17 @@ func (e *groupConcat) UpdatePartialResult(sctx sessionctx.Context, rowsInGroup [
 			if isNull {
 				break
 			}
-			if p.buffer == nil {
-				p.buffer = &bytes.Buffer{}
-			}
-			p.buffer.WriteString(v)
+			p.valsBuf.WriteString(v)
 		}
 		if isNull {
-			if p.buffer != nil {
-				p.buffer.Truncate(preLen)
-			}
 			continue
 		}
+		if p.buffer == nil {
+			p.buffer = &bytes.Buffer{}
+		} else {
+			p.buffer.WriteString(e.sep)
+		}
+		p.buffer.WriteString(p.valsBuf.String())
 	}
 	if p.buffer != nil {
 		return e.truncatePartialResultIfNeed(sctx, p.buffer)
@@ -144,8 +144,7 @@ func (e *groupConcat) GetTruncated() *int32 {
 
 type partialResult4GroupConcatDistinct struct {
 	basePartialResult4GroupConcat
-	valsBuf *bytes.Buffer
-	valSet  set.StringSet
+	valSet set.StringSet
 }
 
 type groupConcatDistinct struct {
diff --git a/executor/aggregate_test.go b/executor/aggregate_test.go
index ee4d1c5b2285c..36bc7cba42371 100644
--- a/executor/aggregate_test.go
+++ b/executor/aggregate_test.go
@@ -407,6 +407,9 @@ func (s *testSuite) TestGroupConcatAggr(c *C) {
 
 	result = tk.MustQuery("select id, group_concat(name SEPARATOR '123') from test group by id order by id")
 	result.Check(testkit.Rows("1 101232012330", "2 20", "3 200123500"))
+
+	// issue #9920
+	tk.MustQuery("select group_concat(123, null)").Check(testkit.Rows("<nil>"))
 }
 
 func (s *testSuite) TestSelectDistinct(c *C) {