diff --git a/docs/05-plugin-development.md b/docs/05-plugin-development.md
index 36035a3a9c1..adab7fceb58 100644
--- a/docs/05-plugin-development.md
+++ b/docs/05-plugin-development.md
@@ -446,7 +446,7 @@ for (const moduleId of this.moduleIds) { /* ... */ }
 
 or converted into an Array via `Array.from(this.moduleIds)`.
 
-#### `this.parse(code: string, acornOptions: AcornOptions) => ESTree.Program`
+#### `this.parse(code: string, acornOptions?: AcornOptions) => ESTree.Program`
 
 Use Rollup's internal acorn instance to parse code to an AST.