diff --git a/src/options.ts b/src/options.ts
index 0a888c83..8f634ccd 100644
--- a/src/options.ts
+++ b/src/options.ts
@@ -30,10 +30,7 @@ export const ERROR_WARNINGS_ENABLED_LANGUAGE_OUT_INVALID =
  * @param format
  * @return boolean
  */
-export const isESMFormat = (format?: ModuleFormat): boolean => {
-  // TODO: remove `| 'esm'` when rollup upgrades its typings
-  return format === 'esm' || format === 'es';
-};
+export const isESMFormat = (format?: ModuleFormat): boolean => format === 'esm' || format === 'es';
 
 /**
  * Throw Errors if compile options will result in unexpected behaviour.
diff --git a/src/transformers/exports.ts b/src/transformers/exports.ts
index eda4e6e5..d58b5f6d 100644
--- a/src/transformers/exports.ts
+++ b/src/transformers/exports.ts
@@ -128,11 +128,7 @@ export default class ExportTransform extends Transform implements TransformInter
    * @return modified input source with window scoped references.
    */
   public async preCompilation(code: string): Promise<TransformSourceDescription> {
-    if (this.outputOptions === null) {
-      this.context.warn(
-        'Rollup Plugin Closure Compiler, OutputOptions not known before Closure Compiler invocation.',
-      );
-    } else if (isESMFormat(this.outputOptions.format)) {
+    if (isESMFormat(this.outputOptions.format)) {
       await this.deriveExports(code);
       const source = new MagicString(code);
 
@@ -171,11 +167,7 @@ export default class ExportTransform extends Transform implements TransformInter
    * @return Promise containing the repaired source
    */
   public async postCompilation(code: string): Promise<TransformSourceDescription> {
-    if (this.outputOptions === null) {
-      this.context.warn(
-        'Rollup Plugin Closure Compiler, OutputOptions not known before Closure Compiler invocation.',
-      );
-    } else if (isESMFormat(this.outputOptions.format)) {
+    if (isESMFormat(this.outputOptions.format)) {
       const source = new MagicString(code);
       const program = parse(code);
       let collectedExportsToAppend: Map<string | null, Array<string>> = new Map();