diff --git a/tests/src/test/java/org/mozilla/javascript/tests/NullishCoalescingOpTest.java b/tests/src/test/java/org/mozilla/javascript/tests/NullishCoalescingOpTest.java
index 4c544c3f6f..8615aa3478 100644
--- a/tests/src/test/java/org/mozilla/javascript/tests/NullishCoalescingOpTest.java
+++ b/tests/src/test/java/org/mozilla/javascript/tests/NullishCoalescingOpTest.java
@@ -9,14 +9,7 @@
 public class NullishCoalescingOpTest {
     @Test
     public void testNullishCoalescingOperatorRequiresES6() {
-        Utils.runWithAllOptimizationLevels(
-                cx -> {
-                    Scriptable scope = cx.initStandardObjects();
-                    assertThrows(
-                            EvaluatorException.class,
-                            () -> cx.evaluateString(scope, "true ?? false", "test.js", 0, null));
-                    return null;
-                });
+        Utils.assertEvaluatorException_1_8("syntax error (test#1)", "true ?? false");
     }
 
     @Test
@@ -75,16 +68,7 @@ public void testNullishCoalescingDoesNotLeakVariables() {
 
     @Test
     public void testNullishAssignmentRequiresES6() {
-        Utils.runWithAllOptimizationLevels(
-                cx -> {
-                    Scriptable scope = cx.initStandardObjects();
-                    assertThrows(
-                            EvaluatorException.class,
-                            () ->
-                                    cx.evaluateString(
-                                            scope, "a = true; a ??= false", "test.js", 0, null));
-                    return null;
-                });
+        Utils.assertEvaluatorException_1_8("syntax error (test#1)", "a = true; a ??= false");
     }
 
     @Test
diff --git a/tests/src/test/java/org/mozilla/javascript/tests/Utils.java b/tests/src/test/java/org/mozilla/javascript/tests/Utils.java
index 0e98321506..a05cb047af 100644
--- a/tests/src/test/java/org/mozilla/javascript/tests/Utils.java
+++ b/tests/src/test/java/org/mozilla/javascript/tests/Utils.java
@@ -227,6 +227,10 @@ private static <T extends Exception> void assertException(
             final Class<T> expectedThrowable,
             final String expectedMessage,
             String js) {
+
+        // to avoid false positives because we use startsWith()
+        assertTrue("expectedMessage can't be empty",expectedMessage != null && !expectedMessage.isEmpty());
+
         Utils.runWithAllOptimizationLevels(
                 cx -> {
                     if (languageVersion > -1) {