diff --git a/rules-tests/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector/Fixture/skip_non_string_arg.php.inc b/rules-tests/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector/Fixture/skip_non_string_arg.php.inc
new file mode 100644
index 00000000..4a4ca308
--- /dev/null
+++ b/rules-tests/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector/Fixture/skip_non_string_arg.php.inc
@@ -0,0 +1,20 @@
+<?php
+
+// skip: non-string argument (enum case) should not crash
+
+namespace Rector\PHPUnit\Tests\PHPUnit120\Rector\Class_\AssertIsTypeMethodCallRector\Fixture;
+
+use PHPUnit\Framework\TestCase;
+
+enum SkipNonStringArgEnum
+{
+    case ONE;
+}
+
+final class SkipNonStringArg extends TestCase
+{
+    public function testMethod(): void
+    {
+        $this->assertThat([], $this->isType(SkipNonStringArgEnum::ONE));
+    }
+}
diff --git a/rules/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector.php b/rules/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector.php
index 7adca290..d9bd9f59 100644
--- a/rules/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector.php
+++ b/rules/PHPUnit120/Rector/Class_/AssertIsTypeMethodCallRector.php
@@ -114,6 +114,10 @@ public function refactor(Node $node): Node|null
         }
 
         $argValue = $this->valueResolver->getValue($arg);
+        if (! is_string($argValue)) {
+            return null;
+        }
+
         if (isset(self::IS_TYPE_VALUE_TO_METHOD[$argValue])) {
             if ($node instanceof MethodCall) {
                 return new MethodCall($node->var, self::IS_TYPE_VALUE_TO_METHOD[$argValue]);