diff --git a/demo/java-demo/src/main/java/com/alibaba/demo/basic/DemoPrivateAccess.java b/demo/java-demo/src/main/java/com/alibaba/demo/basic/DemoPrivateAccess.java
index 92eb513..5d7bbf4 100644
--- a/demo/java-demo/src/main/java/com/alibaba/demo/basic/DemoPrivateAccess.java
+++ b/demo/java-demo/src/main/java/com/alibaba/demo/basic/DemoPrivateAccess.java
@@ -46,7 +46,9 @@ public class DemoPrivateAccess {
      * private member method with arguments
      */
     private String privateFuncWithArgs(List<String> list, String str, int i) {
-        return list.stream().reduce((a, s) -> a + s).orElse("") + " + " + str + " + " + i;
+        return list.stream().reduce((a, s) -> a + s).orElse("")
+                + " + " + (str == null ? "null" : str)
+                + " + " + i;
     }
 
 }
diff --git a/demo/java-demo/src/test/java/com/alibaba/demo/basic/DemoPrivateAccessorTest.java b/demo/java-demo/src/test/java/com/alibaba/demo/basic/DemoPrivateAccessorTest.java
index ab04136..a9dac20 100644
--- a/demo/java-demo/src/test/java/com/alibaba/demo/basic/DemoPrivateAccessorTest.java
+++ b/demo/java-demo/src/test/java/com/alibaba/demo/basic/DemoPrivateAccessorTest.java
@@ -52,7 +52,13 @@ class DemoPrivateAccessorTest {
     void should_use_null_parameter() {
         set(demoPrivateAccess, "pi", null);
         assertNull(get(demoPrivateAccess, "pi"));
-        assertEquals("null + 1", invokeStatic(DemoPrivateAccess.class, "privateStaticFuncWithArgs", null, 1));
+
+        List<String> list = new ArrayList<String>() {{ add("a"); add("b"); add("c"); }};
+        String value = invoke(demoPrivateAccess, "privateFuncWithArgs", list, null, 0);
+        assertEquals("abc + null + 0", value);
+
+        value = invokeStatic(DemoPrivateAccess.class, "privateStaticFuncWithArgs", null, 1);
+        assertEquals("null + 1", value);
     }
 
 }
diff --git a/testable-core/src/main/java/com/alibaba/testable/core/tool/PrivateAccessor.java b/testable-core/src/main/java/com/alibaba/testable/core/tool/PrivateAccessor.java
index 645d7fd..1e08c56 100644
--- a/testable-core/src/main/java/com/alibaba/testable/core/tool/PrivateAccessor.java
+++ b/testable-core/src/main/java/com/alibaba/testable/core/tool/PrivateAccessor.java
@@ -109,10 +109,10 @@ public class PrivateAccessor {
         }
         Class<?> commonClass = cls[0];
         for (int i = 1; i < cls.length; i++) {
-            if (cls[i].isPrimitive()) {
-                return null;
-            } else if (cls[i] == null) {
+            if (cls[i] == null) {
                 continue;
+            } else if (cls[i].isPrimitive()) {
+                return null;
             }
             commonClass = getCommonClassOf(commonClass, cls[i]);
         }