diff --git a/build/lib/rollup.js b/build/lib/rollup.js
index 50e27aed37..e83d01b22d 100644
--- a/build/lib/rollup.js
+++ b/build/lib/rollup.js
@@ -39,8 +39,10 @@ async function rollupModule(options) {
         });
         const result = generatedBundle.output[0];
         result.code = result.code + '\n//# sourceMappingURL=' + path.basename(outputMapName);
-        await fs.promises.writeFile(outputFilePath, result.code);
-        await fs.promises.writeFile(outputMapPath, result.map);
+        await fs.promises.writeFile(outputFilePath, result.code.toString());
+        if (result.map) {
+            await fs.promises.writeFile(outputMapPath, result.map.toString());
+        }
         return {
             name: moduleName,
             result: true
diff --git a/build/lib/rollup.ts b/build/lib/rollup.ts
index 9083ccd254..3aed03a514 100644
--- a/build/lib/rollup.ts
+++ b/build/lib/rollup.ts
@@ -54,8 +54,10 @@ async function rollupModule(options: IRollupOptions) {
 		const result = generatedBundle.output[0];
 		result.code = result.code + '\n//# sourceMappingURL=' + path.basename(outputMapName);
 
-		await fs.promises.writeFile(outputFilePath, result.code);
-		await fs.promises.writeFile(outputMapPath, <any>result.map);
+		await fs.promises.writeFile(outputFilePath, result.code.toString());
+		if (result.map) {
+			await fs.promises.writeFile(outputMapPath, result.map.toString());
+		}
 
 		return {
 			name: moduleName,