diff --git a/src/index.js b/src/index.js
index 9033b64..973f54c 100644
--- a/src/index.js
+++ b/src/index.js
@@ -15,7 +15,7 @@ const {
 } = require("./utils");
 
 const schema = require("./options.json");
-const { minify: minifyWorker } = require("./minify");
+const { minify } = require("./minify");
 
 /** @typedef {import("schema-utils/declarations/validate").Schema} Schema */
 /** @typedef {import("webpack").Compiler} Compiler */
@@ -134,7 +134,7 @@ const { minify: minifyWorker } = require("./minify");
 
 /**
  * @template T
- * @typedef {JestWorker & { transform: (options: string) => InternalResult, minify: (options: InternalOptions<T>) => InternalResult }} MinimizerWorker
+ * @typedef {JestWorker & { transform: (options: string) => Promise<InternalResult>, minify: (options: InternalOptions<T>) => Promise<InternalResult> }} MinimizerWorker
  */
 
 /**
@@ -578,7 +578,7 @@ class CssMinimizerPlugin {
           try {
             result = await (getWorker
               ? getWorker().transform(getSerializeJavascript()(options))
-              : minifyWorker(options));
+              : minify(options));
           } catch (error) {
             const hasSourceMap =
               inputSourceMap && CssMinimizerPlugin.isSourceMap(inputSourceMap);
diff --git a/src/minify.js b/src/minify.js
index 000f0fd..8f76cbf 100644
--- a/src/minify.js
+++ b/src/minify.js
@@ -6,7 +6,7 @@
  * @param {import("./index.js").InternalOptions<T>} options
  * @returns {Promise<InternalResult>}
  */
-const minify = async (options) => {
+async function minify(options) {
   const minifyFns = Array.isArray(options.minimizer.implementation)
     ? options.minimizer.implementation
     : [options.minimizer.implementation];
@@ -59,7 +59,7 @@ const minify = async (options) => {
   }
 
   return result;
-};
+}
 
 /**
  * @param {string} options
diff --git a/src/utils.js b/src/utils.js
index ccef7d4..8466f32 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -69,7 +69,7 @@ function throttleAll(limit, tasks) {
 /* istanbul ignore next */
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
@@ -191,7 +191,7 @@ cssnanoMinify.supportsWorkerThreads = () => true;
 /* istanbul ignore next */
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
@@ -220,7 +220,7 @@ cssoMinify.supportsWorkerThreads = () => true;
 /* istanbul ignore next */
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
@@ -265,7 +265,7 @@ cleanCssMinify.supportsWorkerThreads = () => true;
 /* istanbul ignore next */
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
@@ -359,7 +359,7 @@ esbuildMinify.supportsWorkerThreads = () => false;
 /* istanbul ignore next */
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
@@ -405,7 +405,7 @@ parcelCssMinify.supportsWorkerThreads = () => false;
 /* istanbul ignore next */
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
@@ -451,7 +451,7 @@ lightningCssMinify.supportsWorkerThreads = () => false;
 /* istanbul ignore next */
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
diff --git a/types/index.d.ts b/types/index.d.ts
index 43431b2..c34d48c 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -222,8 +222,8 @@ type BasePluginOptions = {
   parallel?: Parallel;
 };
 type MinimizerWorker<T> = JestWorker & {
-  transform: (options: string) => InternalResult;
-  minify: (options: InternalOptions<T>) => InternalResult;
+  transform: (options: string) => Promise<InternalResult>;
+  minify: (options: InternalOptions<T>) => Promise<InternalResult>;
 };
 type ProcessOptionsExtender =
   | ProcessOptions
diff --git a/types/utils.d.ts b/types/utils.d.ts
index 7d9615d..e1ea35c 100644
--- a/types/utils.d.ts
+++ b/types/utils.d.ts
@@ -25,13 +25,13 @@ export function throttleAll<T>(limit: number, tasks: Task<T>[]): Promise<T[]>;
 export function memoize<T>(fn: (() => any) | undefined): () => T;
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
 export function cssnanoMinify(
   input: Input,
-  sourceMap: RawSourceMap | undefined,
+  sourceMap?: import("@jridgewell/trace-mapping").EncodedSourceMap | undefined,
   minimizerOptions?: import("./index.js").CustomOptions | undefined,
 ): Promise<MinimizedResult>;
 export namespace cssnanoMinify {
@@ -39,13 +39,13 @@ export namespace cssnanoMinify {
 }
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
 export function cssoMinify(
   input: Input,
-  sourceMap: RawSourceMap | undefined,
+  sourceMap?: import("@jridgewell/trace-mapping").EncodedSourceMap | undefined,
   minimizerOptions?: import("./index.js").CustomOptions | undefined,
 ): Promise<MinimizedResult>;
 export namespace cssoMinify {
@@ -53,13 +53,13 @@ export namespace cssoMinify {
 }
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
 export function cleanCssMinify(
   input: Input,
-  sourceMap: RawSourceMap | undefined,
+  sourceMap?: import("@jridgewell/trace-mapping").EncodedSourceMap | undefined,
   minimizerOptions?: import("./index.js").CustomOptions | undefined,
 ): Promise<MinimizedResult>;
 export namespace cleanCssMinify {
@@ -67,13 +67,13 @@ export namespace cleanCssMinify {
 }
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
 export function esbuildMinify(
   input: Input,
-  sourceMap: RawSourceMap | undefined,
+  sourceMap?: import("@jridgewell/trace-mapping").EncodedSourceMap | undefined,
   minimizerOptions?: import("./index.js").CustomOptions | undefined,
 ): Promise<MinimizedResult>;
 export namespace esbuildMinify {
@@ -81,13 +81,13 @@ export namespace esbuildMinify {
 }
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
 export function parcelCssMinify(
   input: Input,
-  sourceMap: RawSourceMap | undefined,
+  sourceMap?: import("@jridgewell/trace-mapping").EncodedSourceMap | undefined,
   minimizerOptions?: import("./index.js").CustomOptions | undefined,
 ): Promise<MinimizedResult>;
 export namespace parcelCssMinify {
@@ -95,13 +95,13 @@ export namespace parcelCssMinify {
 }
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
 export function lightningCssMinify(
   input: Input,
-  sourceMap: RawSourceMap | undefined,
+  sourceMap?: import("@jridgewell/trace-mapping").EncodedSourceMap | undefined,
   minimizerOptions?: import("./index.js").CustomOptions | undefined,
 ): Promise<MinimizedResult>;
 export namespace lightningCssMinify {
@@ -109,13 +109,13 @@ export namespace lightningCssMinify {
 }
 /**
  * @param {Input} input
- * @param {RawSourceMap | undefined} sourceMap
+ * @param {RawSourceMap} [sourceMap]
  * @param {CustomOptions} [minimizerOptions]
  * @return {Promise<MinimizedResult>}
  */
 export function swcMinify(
   input: Input,
-  sourceMap: RawSourceMap | undefined,
+  sourceMap?: import("@jridgewell/trace-mapping").EncodedSourceMap | undefined,
   minimizerOptions?: import("./index.js").CustomOptions | undefined,
 ): Promise<MinimizedResult>;
 export namespace swcMinify {