diff --git a/src/Adapters/CommonMark/CodeBlockRenderer.php b/src/Adapters/CommonMark/CodeBlockRenderer.php
index 4ba1917..a177bde 100644
--- a/src/Adapters/CommonMark/CodeBlockRenderer.php
+++ b/src/Adapters/CommonMark/CodeBlockRenderer.php
@@ -41,11 +41,15 @@ public function render(Node $node, ChildNodeRendererInterface $childRenderer)
     protected function detectGrammar(FencedCode $node): Grammar|string
     {
         if (! isset($node->getInfoWords()[0]) || $node->getInfoWords()[0] === '') {
-            return 'txt';
+            return Grammar::Txt;
         }
 
         preg_match('/[a-zA-Z]+/', $node->getInfoWords()[0], $matches);
 
+        if (! $this->phiki->environment->grammars->has($matches[0])) {
+            return Grammar::Txt;
+        }
+
         return $matches[0];
     }
 }
diff --git a/tests/.pest/snapshots/Adapters/CommonMark/PhikiExtensionTest/it_falls_back_to_txt_if_the_grammar_doesnt_exist.snap b/tests/.pest/snapshots/Adapters/CommonMark/PhikiExtensionTest/it_falls_back_to_txt_if_the_grammar_doesnt_exist.snap
new file mode 100644
index 0000000..0dd5421
--- /dev/null
+++ b/tests/.pest/snapshots/Adapters/CommonMark/PhikiExtensionTest/it_falls_back_to_txt_if_the_grammar_doesnt_exist.snap
@@ -0,0 +1,2 @@
+<pre class="phiki github-dark" style="background-color: #24292e;color: #e1e4e8;"><code><span class="line"><span class="token">class A {}
+</span></span></code></pre>
diff --git a/tests/Adapters/CommonMark/PhikiExtensionTest.php b/tests/Adapters/CommonMark/PhikiExtensionTest.php
index 1e6e8f3..b4aed43 100644
--- a/tests/Adapters/CommonMark/PhikiExtensionTest.php
+++ b/tests/Adapters/CommonMark/PhikiExtensionTest.php
@@ -65,3 +65,21 @@ class A {}
 
     expect($generated)->toMatchSnapshot();
 });
+
+it('falls back to txt if the grammar doesnt exist', function () {
+    $environment = new Environment;
+
+    $environment
+        ->addExtension(new CommonMarkCoreExtension)
+        ->addExtension(new PhikiExtension('github-dark'));
+
+    $markdown = new MarkdownConverter($environment);
+
+    $generated = $markdown->convert(<<<'MD'
+    ```nonexistentlang
+    class A {}
+    ```
+    MD);
+
+    expect($generated)->toMatchSnapshot();
+});