diff --git a/helper/helper_test.go b/helper/helper_test.go
index 4bc98b5..27c3171 100644
--- a/helper/helper_test.go
+++ b/helper/helper_test.go
@@ -100,8 +100,8 @@ func testHelper(t *testing.T,
 			}
 		})
 
-		if got := stdout.String(); !strings.HasPrefix(got, wantPayload) {
-			t.Errorf("Start: stdout = %v, want %v",
+		if got := stderr.String(); got != wantPayload {
+			t.Errorf("Start: stderr = %v, want %v",
 				got, wantPayload)
 		}
 	})
@@ -123,8 +123,8 @@ func testHelper(t *testing.T,
 				err, stdout, stderr)
 		}
 
-		if got := stdout.String(); !strings.HasPrefix(got, wantPayload) {
-			t.Errorf("Start() stdout = %v, want %v",
+		if got := stderr.String(); got != wantPayload {
+			t.Errorf("Start() stderr = %v, want %v",
 				got, wantPayload)
 		}
 	})
diff --git a/helper/stub.go b/helper/stub.go
index f64263c..988004c 100644
--- a/helper/stub.go
+++ b/helper/stub.go
@@ -74,7 +74,7 @@ func flagRestoreFiles(offset int, ap, sp string) (argsFile, statFile *os.File) {
 func genericStub(argsFile, statFile *os.File) {
 	if argsFile != nil {
 		// this output is checked by parent
-		if _, err := io.Copy(os.Stdout, argsFile); err != nil {
+		if _, err := io.Copy(os.Stderr, argsFile); err != nil {
 			panic("cannot read args: " + err.Error())
 		}
 	}