diff --git a/src/minify.js b/src/minify.js
index bde0915..9bffb17 100644
--- a/src/minify.js
+++ b/src/minify.js
@@ -49,7 +49,7 @@ const minify = async (options) => {
 
   return {
     code: result.css,
-    map: result.map,
+    map: result.map && result.map.toString(),
     warnings: warningsToString(result.warnings()),
   };
 };
diff --git a/test/__snapshots__/sourceMap-option.test.js.snap.webpack4 b/test/__snapshots__/sourceMap-option.test.js.snap.webpack4
index fb5a807..308f73d 100644
--- a/test/__snapshots__/sourceMap-option.test.js.snap.webpack4
+++ b/test/__snapshots__/sourceMap-option.test.js.snap.webpack4
@@ -69,6 +69,92 @@ exports[`when applied with "sourceMap" option should work with SourceMapDevToolP
 
 exports[`when applied with "sourceMap" option should work with SourceMapDevToolPlugin plugin): warnings 1`] = `Array []`;
 
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "false" value: assets 1`] = `
+Object {
+  "entry.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry.css.map*/",
+  "entry.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.scss"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCCnB,OAGI,iBAAkB",
+    "file": "entry.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n  a {\\n    text-align: center;\\n  }\\n}"
+    ],
+    "sourceRoot": ""
+},
+  "entry2.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry2.css.map*/",
+  "entry2.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.css"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCKnB,OACE,iBAAkB",
+    "file": "entry2.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n}\\n\\nbody a {\\n  text-align: center;\\n}"
+    ],
+    "sourceRoot": ""
+},
+}
+`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "false" value: errors 1`] = `Array []`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "false" value: warnings 1`] = `Array []`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "true" value: assets 1`] = `
+Object {
+  "entry.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry.css.map*/",
+  "entry.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.scss"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCCnB,OAGI,iBAAkB",
+    "file": "entry.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n  a {\\n    text-align: center;\\n  }\\n}"
+    ],
+    "sourceRoot": ""
+},
+  "entry2.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry2.css.map*/",
+  "entry2.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.css"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCKnB,OACE,iBAAkB",
+    "file": "entry2.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n}\\n\\nbody a {\\n  text-align: center;\\n}"
+    ],
+    "sourceRoot": ""
+},
+}
+`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "true" value: errors 1`] = `Array []`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "true" value: warnings 1`] = `Array []`;
+
 exports[`when applied with "sourceMap" option should work with the "devtool" option: assets 1`] = `
 Object {
   "entry.css": "body{font-weight:700;color:red}body a{text-align:center}
diff --git a/test/__snapshots__/sourceMap-option.test.js.snap.webpack5 b/test/__snapshots__/sourceMap-option.test.js.snap.webpack5
index 48a445b..bef69bc 100644
--- a/test/__snapshots__/sourceMap-option.test.js.snap.webpack5
+++ b/test/__snapshots__/sourceMap-option.test.js.snap.webpack5
@@ -69,6 +69,92 @@ exports[`when applied with "sourceMap" option should work with SourceMapDevToolP
 
 exports[`when applied with "sourceMap" option should work with SourceMapDevToolPlugin plugin): warnings 1`] = `Array []`;
 
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "false" value: assets 1`] = `
+Object {
+  "entry.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry.css.map*/",
+  "entry.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.scss"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCCnB,OAGI,iBAAkB",
+    "file": "entry.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n  a {\\n    text-align: center;\\n  }\\n}"
+    ],
+    "sourceRoot": ""
+},
+  "entry2.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry2.css.map*/",
+  "entry2.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.css"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCKnB,OACE,iBAAkB",
+    "file": "entry2.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n}\\n\\nbody a {\\n  text-align: center;\\n}"
+    ],
+    "sourceRoot": ""
+},
+}
+`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "false" value: errors 1`] = `Array []`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "false" value: warnings 1`] = `Array []`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "true" value: assets 1`] = `
+Object {
+  "entry.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry.css.map*/",
+  "entry.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.scss"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCCnB,OAGI,iBAAkB",
+    "file": "entry.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n  a {\\n    text-align: center;\\n  }\\n}"
+    ],
+    "sourceRoot": ""
+},
+  "entry2.css": "body{font-weight:700;color:red}body a{text-align:center}
+/*# sourceMappingURL=entry2.css.map*/",
+  "entry2.css.map": {
+    "version": 3,
+    "sources": [
+        "webpack:///./sourcemap/bar.scss",
+        "webpack:///./sourcemap/foo.css"
+    ],
+    "names": [],
+    "mappings": "AAAA,KACE,eAAiB,CCEjB,SDFiB,CCKnB,OACE,iBAAkB",
+    "file": "entry2.css",
+    "sourcesContent": [
+        "body {\\n  font-weight: bold;\\n}",
+        "@import 'bar';\\n\\nbody {\\n  color: red;\\n}\\n\\nbody a {\\n  text-align: center;\\n}"
+    ],
+    "sourceRoot": ""
+},
+}
+`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "true" value: errors 1`] = `Array []`;
+
+exports[`when applied with "sourceMap" option should work with the "devtool" option and the "parallel" option with "true" value: warnings 1`] = `Array []`;
+
 exports[`when applied with "sourceMap" option should work with the "devtool" option: assets 1`] = `
 Object {
   "entry.css": "body{font-weight:700;color:red}body a{text-align:center}
diff --git a/test/__snapshots__/worker.test.js.snap.webpack4 b/test/__snapshots__/worker.test.js.snap.webpack4
index 24006ff..62d9fc4 100644
--- a/test/__snapshots__/worker.test.js.snap.webpack4
+++ b/test/__snapshots__/worker.test.js.snap.webpack4
@@ -8,43 +8,11 @@ Array [
 
 exports[`worker should minify css: css 1`] = `".foo{color:red}.bar{color:coral}"`;
 
-exports[`worker should minify css: map 1`] = `
-Object {
-  "file": "entry.css",
-  "mappings": "AAAA,KAAK,SAAA,CCCL,KAAK,WAAY",
-  "names": Array [],
-  "sources": Array [
-    "foo.css",
-    "entry.css",
-  ],
-  "sourcesContent": Array [
-    ".foo{color:red;}",
-    ".foo{color:red;}
-.bar{color:coral;}",
-  ],
-  "version": 3,
-}
-`;
+exports[`worker should minify css: map 1`] = `"{\\"version\\":3,\\"sources\\":[\\"foo.css\\",\\"entry.css\\"],\\"names\\":[],\\"mappings\\":\\"AAAA,KAAK,SAAA,CCCL,KAAK,WAAY\\",\\"file\\":\\"entry.css\\",\\"sourcesContent\\":[\\".foo{color:red;}\\",\\".foo{color:red;}\\\\n.bar{color:coral;}\\"]}"`;
 
 exports[`worker should work inputSourceMap as prev: css 1`] = `".foo{color:red}.bar{color:coral}"`;
 
-exports[`worker should work inputSourceMap as prev: map 1`] = `
-Object {
-  "file": "entry.css",
-  "mappings": "AAAA,KAAK,SAAA,CCCL,KAAK,WAAY",
-  "names": Array [],
-  "sources": Array [
-    "foo.css",
-    "entry.css",
-  ],
-  "sourcesContent": Array [
-    ".foo{color:red;}",
-    ".foo{color:red;}
-.bar{color:coral;}",
-  ],
-  "version": 3,
-}
-`;
+exports[`worker should work inputSourceMap as prev: map 1`] = `"{\\"version\\":3,\\"sources\\":[\\"foo.css\\",\\"entry.css\\"],\\"names\\":[],\\"mappings\\":\\"AAAA,KAAK,SAAA,CCCL,KAAK,WAAY\\",\\"file\\":\\"entry.css\\",\\"sourcesContent\\":[\\".foo{color:red;}\\",\\".foo{color:red;}\\\\n.bar{color:coral;}\\"]}"`;
 
 exports[`worker should work options.minify function: css 1`] = `".minify {};"`;
 
diff --git a/test/__snapshots__/worker.test.js.snap.webpack5 b/test/__snapshots__/worker.test.js.snap.webpack5
index 24006ff..62d9fc4 100644
--- a/test/__snapshots__/worker.test.js.snap.webpack5
+++ b/test/__snapshots__/worker.test.js.snap.webpack5
@@ -8,43 +8,11 @@ Array [
 
 exports[`worker should minify css: css 1`] = `".foo{color:red}.bar{color:coral}"`;
 
-exports[`worker should minify css: map 1`] = `
-Object {
-  "file": "entry.css",
-  "mappings": "AAAA,KAAK,SAAA,CCCL,KAAK,WAAY",
-  "names": Array [],
-  "sources": Array [
-    "foo.css",
-    "entry.css",
-  ],
-  "sourcesContent": Array [
-    ".foo{color:red;}",
-    ".foo{color:red;}
-.bar{color:coral;}",
-  ],
-  "version": 3,
-}
-`;
+exports[`worker should minify css: map 1`] = `"{\\"version\\":3,\\"sources\\":[\\"foo.css\\",\\"entry.css\\"],\\"names\\":[],\\"mappings\\":\\"AAAA,KAAK,SAAA,CCCL,KAAK,WAAY\\",\\"file\\":\\"entry.css\\",\\"sourcesContent\\":[\\".foo{color:red;}\\",\\".foo{color:red;}\\\\n.bar{color:coral;}\\"]}"`;
 
 exports[`worker should work inputSourceMap as prev: css 1`] = `".foo{color:red}.bar{color:coral}"`;
 
-exports[`worker should work inputSourceMap as prev: map 1`] = `
-Object {
-  "file": "entry.css",
-  "mappings": "AAAA,KAAK,SAAA,CCCL,KAAK,WAAY",
-  "names": Array [],
-  "sources": Array [
-    "foo.css",
-    "entry.css",
-  ],
-  "sourcesContent": Array [
-    ".foo{color:red;}",
-    ".foo{color:red;}
-.bar{color:coral;}",
-  ],
-  "version": 3,
-}
-`;
+exports[`worker should work inputSourceMap as prev: map 1`] = `"{\\"version\\":3,\\"sources\\":[\\"foo.css\\",\\"entry.css\\"],\\"names\\":[],\\"mappings\\":\\"AAAA,KAAK,SAAA,CCCL,KAAK,WAAY\\",\\"file\\":\\"entry.css\\",\\"sourcesContent\\":[\\".foo{color:red;}\\",\\".foo{color:red;}\\\\n.bar{color:coral;}\\"]}"`;
 
 exports[`worker should work options.minify function: css 1`] = `".minify {};"`;
 
diff --git a/test/sourceMap-option.test.js b/test/sourceMap-option.test.js
index 5110322..e2bebb0 100644
--- a/test/sourceMap-option.test.js
+++ b/test/sourceMap-option.test.js
@@ -70,6 +70,38 @@ describe('when applied with "sourceMap" option', () => {
     expect(getWarnings(stats)).toMatchSnapshot('warnings');
   });
 
+  it('should work with the "devtool" option and the "parallel" option with "false" value', async () => {
+    const compiler = getCompiler(baseConfig);
+
+    new CssMinimizerPlugin({
+      parallel: false,
+    }).apply(compiler);
+
+    const stats = await compile(compiler);
+
+    expect(readAssets(compiler, stats, /\.css(\.map)?$/)).toMatchSnapshot(
+      'assets'
+    );
+    expect(getErrors(stats)).toMatchSnapshot('errors');
+    expect(getWarnings(stats)).toMatchSnapshot('warnings');
+  });
+
+  it('should work with the "devtool" option and the "parallel" option with "true" value', async () => {
+    const compiler = getCompiler(baseConfig);
+
+    new CssMinimizerPlugin({
+      parallel: true,
+    }).apply(compiler);
+
+    const stats = await compile(compiler);
+
+    expect(readAssets(compiler, stats, /\.css(\.map)?$/)).toMatchSnapshot(
+      'assets'
+    );
+    expect(getErrors(stats)).toMatchSnapshot('errors');
+    expect(getWarnings(stats)).toMatchSnapshot('warnings');
+  });
+
   it('should work with the "true" value', async () => {
     const config = Object.assign(baseConfig, {
       module: {