diff --git a/README.md b/README.md
index 0b97979..a8230bb 100644
--- a/README.md
+++ b/README.md
@@ -549,10 +549,10 @@ Type:
 ```ts
 type transform =
   | {
-      transformer: (input: string, absoluteFilename: string) => string;
+      transformer: (input: string, absoluteFilename: string) => string | Buffer;
       cache?: boolean | TransformerCacheObject | undefined;
     }
-  | ((input: string, absoluteFilename: string) => string);
+  | ((input: string, absoluteFilename: string) => string | Buffer);
 ```
 
 Default: `undefined`
diff --git a/src/index.js b/src/index.js
index a1ff362..efb4505 100644
--- a/src/index.js
+++ b/src/index.js
@@ -74,6 +74,7 @@ const template = /\[\\*([\w:]+)\\*\]/i;
  * @callback TransformerFunction
  * @param {Buffer} input
  * @param {string} absoluteFilename
+ * @returns {string | Buffer}
  */
 
 /**
diff --git a/types/index.d.ts b/types/index.d.ts
index 51a1723..9d310be 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -49,6 +49,7 @@ export = CopyPlugin;
  * @callback TransformerFunction
  * @param {Buffer} input
  * @param {string} absoluteFilename
+ * @returns {string | Buffer}
  */
 /**
  * @typedef {{ keys: { [key: string]: any } } | { keys: ((defaultCacheKeys: { [key: string]: any }, absoluteFilename: string) => Promise<{ [key: string]: any }>) }} TransformerCacheObject
@@ -229,7 +230,10 @@ type ToFunction = (pathData: {
 }) => string;
 type To = string | ToFunction;
 type ToType = "dir" | "file" | "template";
-type TransformerFunction = (input: Buffer, absoluteFilename: string) => any;
+type TransformerFunction = (
+  input: Buffer,
+  absoluteFilename: string
+) => string | Buffer;
 type TransformerCacheObject =
   | {
       keys: {