diff --git a/index.d.ts b/index.d.ts
index 4489af8..439f0db 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -75,10 +75,10 @@ declare namespace Bowser {
       /**
        * Check if the version is equals the browser version
        * @param version The string to compare with the browser version
-       * @returns {boolean}
+       * @returns {boolean | undefined} Undefined if no browser version is available.
        */
 
-      compareVersion(version: string): boolean;
+      compareVersion(version: string): boolean | undefined;
 
       /**
        * Get parsed browser object
diff --git a/test/types/fixtures/consumer-cjs.ts b/test/types/fixtures/consumer-cjs.ts
index 2414e48..23d66ab 100644
--- a/test/types/fixtures/consumer-cjs.ts
+++ b/test/types/fixtures/consumer-cjs.ts
@@ -9,6 +9,9 @@ const result: Bowser.Parser.ParsedResult = Bowser.parse(UA);
 const parser: Bowser.Parser.Parser = Bowser.getParser(UA);
 const name: string | undefined = result.browser.name;
 const satisfies: boolean | undefined = parser.satisfies({ chrome: '>100' });
+const comparison: boolean | undefined = parser.compareVersion('>100');
+// @ts-expect-error A browser without a detected version returns undefined.
+const definiteComparison: boolean = parser.compareVersion('>100');
 const hints: Bowser.ClientHints = { mobile: false };
 const maps: Record<string, string> = Bowser.BROWSER_MAP;
 
diff --git a/test/types/fixtures/consumer-esm.ts b/test/types/fixtures/consumer-esm.ts
index 924bf53..09e3323 100644
--- a/test/types/fixtures/consumer-esm.ts
+++ b/test/types/fixtures/consumer-esm.ts
@@ -12,6 +12,9 @@ const result: ParsedResult = parse(UA);
 const parser: Parser = getParser(UA);
 const tree: checkTree = { chrome: '>100' };
 const satisfies: boolean | undefined = parser.satisfies(tree);
+const comparison: boolean | undefined = parser.compareVersion('>100');
+// @ts-expect-error A browser without a detected version returns undefined.
+const definiteComparison: boolean = parser.compareVersion('>100');
 const hints: ClientHints = { mobile: false };
 const maps: Array<Record<string, string>> = [
   Bowser.BROWSER_MAP, BROWSER_MAP, ENGINE_MAP, OS_MAP, PLATFORMS_MAP,