diff --git a/.changeset/rich-squids-fold.md b/.changeset/rich-squids-fold.md
new file mode 100644
index 0000000000..947bd67a68
--- /dev/null
+++ b/.changeset/rich-squids-fold.md
@@ -0,0 +1,5 @@
+---
+'@tiptap/extension-code-block-lowlight': patch
+---
+
+Fixed a runtime error when initializing `CodeBlockLowlight` by switching the `CodeBlock` import to a named export. This prevents `extend is not a function` errors caused by ESM/CJS interop issues.
diff --git a/packages/extension-code-block-lowlight/src/code-block-lowlight.ts b/packages/extension-code-block-lowlight/src/code-block-lowlight.ts
index d06240630f..3a68721307 100644
--- a/packages/extension-code-block-lowlight/src/code-block-lowlight.ts
+++ b/packages/extension-code-block-lowlight/src/code-block-lowlight.ts
@@ -1,5 +1,5 @@
 import type { CodeBlockOptions } from '@tiptap/extension-code-block'
-import CodeBlock from '@tiptap/extension-code-block'
+import { CodeBlock } from '@tiptap/extension-code-block'
 
 import { LowlightPlugin } from './lowlight-plugin.js'