diff --git a/src/internal/singleflight/singleflight_test.go b/src/internal/singleflight/singleflight_test.go
index 890ab62e03..c0ec0240c7 100644
--- a/src/internal/singleflight/singleflight_test.go
+++ b/src/internal/singleflight/singleflight_test.go
@@ -42,21 +42,30 @@ func TestDoErr(t *testing.T) {
 
 func TestDoDupSuppress(t *testing.T) {
 	var g Group
+	var wg1, wg2 sync.WaitGroup
 	c := make(chan string, 1)
 	var calls int32
 	fn := func() (interface{}, error) {
-		atomic.AddInt32(&calls, 1)
+		if atomic.AddInt32(&calls, 1) == 1 {
+			// First invocation.
+			wg1.Done()
+		}
 		v := <-c
 		c <- v // pump; make available for any future calls
+
+		time.Sleep(10 * time.Millisecond) // let more goroutines enter Do
+
 		return v, nil
 	}
 
 	const n = 10
-	var wg sync.WaitGroup
+	wg1.Add(1)
 	for i := 0; i < n; i++ {
-		wg.Add(1)
+		wg1.Add(1)
+		wg2.Add(1)
 		go func() {
-			defer wg.Done()
+			defer wg2.Done()
+			wg1.Done()
 			v, err, _ := g.Do("key", fn)
 			if err != nil {
 				t.Errorf("Do error: %v", err)
@@ -67,10 +76,12 @@ func TestDoDupSuppress(t *testing.T) {
 			}
 		}()
 	}
-	time.Sleep(10 * time.Millisecond) // let some goroutines above block in Do
+	wg1.Wait()
+	// At least one goroutine is in fn now and all of them have at
+	// least reached the line before the Do.
 	c <- "bar"
-	wg.Wait()
+	wg2.Wait()
 	if got := atomic.LoadInt32(&calls); got <= 0 || got >= n {
-		t.Errorf("number of calls = %d; want over 0 and less than n", got)
+		t.Errorf("number of calls = %d; want over 0 and less than %d", got, n)
 	}
 }