diff --git a/backend/api_test.go b/backend/api_test.go
index 33e7082..eff4f61 100644
--- a/backend/api_test.go
+++ b/backend/api_test.go
@@ -594,6 +594,11 @@ func TestCompleteTask(t *testing.T) {
 	}
 }
 
+func TestCompleteTaskInvalidId(t *testing.T) {
+	e := startServer(t)
+	e.POST("/task/999/complete").Expect().Status(404)
+}
+
 func getDelta(base time.Time, delta float64) (time.Time, time.Time) {
 	start := base.Add(-time.Duration(delta) * time.Second)
 	end := base.Add(time.Duration(delta) * time.Second)