diff --git a/README.md b/README.md
index 28d841d..436dbfe 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@ console.log(["item20", "item19", "item1", "item10", "item2"].sort(compareFn));
 
 [CodeSandbox](https://codesandbox.io/s/alphanum-compare-demo-bfhln)
 
-### `compareFn(a: string, b: string, opts?: Options): number`
+### `compareFn(a: number | string, b: number | string, opts?: Options): number`
 
 It returns a negative value if first argument is less than second argument, zero if they're equal and a positive value otherwise.
 
diff --git a/src/index.spec.ts b/src/index.spec.ts
index 9b56b2d..6ba892d 100644
--- a/src/index.spec.ts
+++ b/src/index.spec.ts
@@ -363,9 +363,7 @@ describe("sorting", function () {
   tests.forEach((test) => {
     it(test.message, () => {
       assert.deepEqual(
-        test.fixture.sort((a, b) =>
-          compareFn(a.toString(), b.toString(), test.options)
-        ),
+        test.fixture.sort((a, b) => compareFn(a, b, test.options)),
         test.expected
       );
     });
diff --git a/src/index.ts b/src/index.ts
index 002b3cc..2d6a464 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -15,15 +15,15 @@ function isSign(code: number) {
 }
 
 function compare(
-  a: string,
-  b: string,
+  a: number | string,
+  b: number | string,
   opts?: { insensitive?: boolean; sign?: boolean }
 ): number {
   const checkCase = opts?.insensitive ?? false;
   const checkSign = opts?.sign ?? false;
 
-  const av = checkCase ? a.toLowerCase() : a;
-  const bv = checkCase ? b.toLowerCase() : b;
+  const av = checkCase ? `${a}`.toLowerCase() : `${a}`;
+  const bv = checkCase ? `${b}`.toLowerCase() : `${b}`;
   let ia = 0;
   let ib = 0;
   const ma = av.length;