diff --git a/src/Instance.ts b/src/Instance.ts
index 16e2c060e8..7aad1daaca 100644
--- a/src/Instance.ts
+++ b/src/Instance.ts
@@ -21,8 +21,8 @@ export class Marked {
   defaults = _getDefaults();
   options = this.setOptions;
 
-  parse = this.#parseMarkdown(_Lexer.lex, _Parser.parse);
-  parseInline = this.#parseMarkdown(_Lexer.lexInline, _Parser.parseInline);
+  parse = this.parseMarkdown(_Lexer.lex, _Parser.parse);
+  parseInline = this.parseMarkdown(_Lexer.lexInline, _Parser.parseInline);
 
   Parser = _Parser;
   Renderer = _Renderer;
@@ -514,22 +514,25 @@ export class Marked {
     return _Parser.parse(tokens, options ?? this.defaults);
   }
 
-  #parseMarkdown(lexer: (src: string, options?: MarkedOptions) => TokensList | Token[], parser: (tokens: Token[], options?: MarkedOptions) => string) {
-    return (src: string, options?: MarkedOptions | undefined | null): string | Promise<string> => {
+  private parseMarkdown(lexer: (src: string, options?: MarkedOptions) => TokensList | Token[], parser: (tokens: Token[], options?: MarkedOptions) => string) {
+    type overloadedParse = {
+      (src: string, options: MarkedOptions & { async: true }): Promise<string>;
+      (src: string, options: MarkedOptions & { async: false }): string;
+      (src: string, options?: MarkedOptions | undefined | null): string | Promise<string>;
+    };
+
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const parse: overloadedParse = (src: string, options?: MarkedOptions | undefined | null): any => {
       const origOpt = { ...options };
       const opt = { ...this.defaults, ...origOpt };
 
-      // Show warning if an extension set async to true but the parse was called with async: false
-      if (this.defaults.async === true && origOpt.async === false) {
-        if (!opt.silent) {
-          console.warn('marked(): The async option was set to true by an extension. The async: false option sent to parse will be ignored.');
-        }
+      const throwError = this.onError(!!opt.silent, !!opt.async);
 
-        opt.async = true;
+      // throw error if an extension set async to true but parse was called with async: false
+      if (this.defaults.async === true && origOpt.async === false) {
+        return throwError(new Error('marked(): The async option was set to true by an extension. Remove async: false from the parse options object to return a Promise.'));
       }
 
-      const throwError = this.#onError(!!opt.silent, !!opt.async);
-
       // throw error in case of non string input
       if (typeof src === 'undefined' || src === null) {
         return throwError(new Error('marked(): input parameter is undefined or null'));
@@ -573,9 +576,11 @@ export class Marked {
         return throwError(e as Error);
       }
     };
+
+    return parse;
   }
 
-  #onError(silent: boolean, async: boolean) {
+  private onError(silent: boolean, async: boolean) {
     return (e: Error): string | Promise<string> => {
       e.message += '\nPlease report this to https://github.com/markedjs/marked.';
 
diff --git a/src/marked.ts b/src/marked.ts
index ac83640ae5..7ec18f5ae6 100644
--- a/src/marked.ts
+++ b/src/marked.ts
@@ -32,8 +32,10 @@ export function marked(src: string, options: MarkedOptions & { async: true }): P
  * @param options Optional hash of options
  * @return String of compiled HTML. Will be a Promise of string if async is set to true by any extensions.
  */
-export function marked(src: string, options?: MarkedOptions): string | Promise<string>;
-export function marked(src: string, opt?: MarkedOptions): string | Promise<string> {
+export function marked(src: string, options: MarkedOptions & { async: false }): string;
+export function marked(src: string, options: MarkedOptions & { async: true }): Promise<string>;
+export function marked(src: string, options?: MarkedOptions | undefined | null): string | Promise<string>;
+export function marked(src: string, opt?: MarkedOptions | undefined | null): string | Promise<string> {
   return markedInstance.parse(src, opt);
 }
 
diff --git a/test/types/marked.ts b/test/types/marked.ts
index 607fbf50c8..bf7a3e9433 100644
--- a/test/types/marked.ts
+++ b/test/types/marked.ts
@@ -261,7 +261,6 @@ marked.use(asyncExtension);
 const md = '# foobar';
 const asyncMarked: string = await marked(md, { async: true });
 const promiseMarked: Promise<string> = marked(md, { async: true });
-// @ts-expect-error marked can still be async if an extension sets `async: true`
 const notAsyncMarked: string = marked(md, { async: false });
 // @ts-expect-error marked can still be async if an extension sets `async: true`
 const defaultMarked: string = marked(md);
@@ -270,7 +269,6 @@ const stringMarked: string = marked(md) as string;
 
 const asyncMarkedParse: string = await marked.parse(md, { async: true });
 const promiseMarkedParse: Promise<string> = marked.parse(md, { async: true });
-// @ts-expect-error marked can still be async if an extension sets `async: true`
 const notAsyncMarkedParse: string = marked.parse(md, { async: false });
 // @ts-expect-error marked can still be async if an extension sets `async: true`
 const defaultMarkedParse: string = marked.parse(md);
diff --git a/test/unit/marked.test.js b/test/unit/marked.test.js
index 6d2854f299..87931c86a0 100644
--- a/test/unit/marked.test.js
+++ b/test/unit/marked.test.js
@@ -630,10 +630,10 @@ used extension2 walked</p>
       assert.strictEqual(typeof marked.parse('test', { async: false }), 'string');
     });
 
-    it('should return Promise if async is set by extension', () => {
+    it('should throw an error if async is set by extension', () => {
       marked.use({ async: true });
 
-      assert.ok(marked.parse('test', { async: false }) instanceof Promise);
+      assert.throws(() => marked.parse('test', { async: false }));
     });
 
     it('should allow deleting/editing tokens', () => {