diff --git a/src/Pug/Symfony/Traits/HelpersHandler.php b/src/Pug/Symfony/Traits/HelpersHandler.php
index b2af19c..724e974 100644
--- a/src/Pug/Symfony/Traits/HelpersHandler.php
+++ b/src/Pug/Symfony/Traits/HelpersHandler.php
@@ -196,9 +196,13 @@ protected function interpolateTwigFunctions(string $code): string
         for ($index = 0; $index < $count; $index++) {
             $token = $tokens[$index];
 
-            if (is_array($token) && $token[0] === T_STRING && $tokens[$index + 1] === '(') {
+            if (is_array($token) && $token[0] === T_STRING && ($tokens[$index + 1] ?? null) === '(') {
                 if ($token[1] === 'function_exists') {
-                    if ($tokens[$index + 3] === ')' && is_array($tokens[$index + 2]) && $tokens[$index + 2][0] === T_CONSTANT_ENCAPSED_STRING && isset($this->twigHelpers[substr($tokens[$index + 2][1], 1, -1)])) {
+                    if ($tokens[$index + 3] === ')' &&
+                        is_array($tokens[$index + 2]) &&
+                        $tokens[$index + 2][0] === T_CONSTANT_ENCAPSED_STRING &&
+                        isset($this->twigHelpers[substr($tokens[$index + 2][1], 1, -1)])
+                    ) {
                         $output .= 'true';
                         $index += 3;
                         continue;
diff --git a/tests/Pug/PugSymfonyBundle/Command/AssetsPublishCommandTest.php b/tests/Pug/PugSymfonyBundle/Command/AssetsPublishCommandTest.php
index 16f4b05..0b8ff09 100644
--- a/tests/Pug/PugSymfonyBundle/Command/AssetsPublishCommandTest.php
+++ b/tests/Pug/PugSymfonyBundle/Command/AssetsPublishCommandTest.php
@@ -45,7 +45,7 @@ public function testCommand()
         file_put_contents($customHelperFile, $customHelper);
         file_put_contents($stylePhpFile, $stylePhp);
 
-        $this->assertStringContainsString('15 templates cached', $output, 'All templates can be cached except filter.pug as the upper filter does not exists.');
+        $this->assertStringContainsString('16 templates cached', $output, 'All templates can be cached except filter.pug as the upper filter does not exists.');
         $this->assertStringContainsString('1 templates failed to be cached', $output, 'filter.pug fails as the upper filter does not exists.');
         $this->assertRegExp('/(Unknown\sfilter\supper|upper:\sFilter\sdoes\s?n[\'o]t\sexists)/', $output, 'filter.pug fails as the upper filter does not exists.');
         $this->assertStringContainsString('filter.pug', $output, 'filter.pug fails as the upper filter does not exists.');
diff --git a/tests/Pug/PugSymfonyEngineTest.php b/tests/Pug/PugSymfonyEngineTest.php
index 5db3e28..7406ba9 100644
--- a/tests/Pug/PugSymfonyEngineTest.php
+++ b/tests/Pug/PugSymfonyEngineTest.php
@@ -146,6 +146,18 @@ public function testPreRenderPhp()
         );
     }
 
+    public function testMixin()
+    {
+        $pugSymfony = new PugSymfonyEngine(self::$kernel);
+        $pugSymfony->setOption('expressionLanguage', 'js');
+        $pugSymfony->setOption('prettyprint', false);
+
+        self::assertSame(
+            '<ul><li class="pet">cat</li><li class="pet">dog</li><li class="pet">pig</li></ul>',
+            $pugSymfony->render('mixin.pug')
+        );
+    }
+
     /**
      * @throws ErrorException
      */
diff --git a/tests/project-s5/templates/mixin.pug b/tests/project-s5/templates/mixin.pug
new file mode 100644
index 0000000..dbdfaa6
--- /dev/null
+++ b/tests/project-s5/templates/mixin.pug
@@ -0,0 +1,6 @@
+mixin pet(name)
+    li.pet= name
+ul
+    +pet('cat')
+    +pet('dog')
+    +pet('pig')