diff --git a/container/intsets/sparse.go b/container/intsets/sparse.go
index be4c5a66cf..be14d0559c 100644
--- a/container/intsets/sparse.go
+++ b/container/intsets/sparse.go
@@ -690,7 +690,7 @@ func (s *Sparse) String() string {
 	buf.WriteByte('{')
 	s.forEach(func(x int) {
 		if buf.Len() > 1 {
-			buf.WriteString(", ")
+			buf.WriteByte(' ')
 		}
 		fmt.Fprintf(&buf, "%d", x)
 	})
diff --git a/container/intsets/sparse_test.go b/container/intsets/sparse_test.go
index 94d2e462fe..cd814c7aa0 100644
--- a/container/intsets/sparse_test.go
+++ b/container/intsets/sparse_test.go
@@ -43,8 +43,8 @@ func TestBasics(t *testing.T) {
 	if !s.Insert(435) {
 		t.Errorf("Insert(435): got false, want true")
 	}
-	if s := s.String(); s != "{3, 435}" {
-		t.Errorf("String({3, 435}): got %q, want \"{3, 435}\"", s)
+	if s := s.String(); s != "{3 435}" {
+		t.Errorf("String({3 435}): got %q, want \"{3 435}\"", s)
 	}
 	if max := s.Max(); max != 435 {
 		t.Errorf("Max: got %d, want 435", max)