diff --git a/src/typeUtils.ts b/src/typeUtils.ts
index b0da5dd7..66170644 100644
--- a/src/typeUtils.ts
+++ b/src/typeUtils.ts
@@ -39,11 +39,11 @@ type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;
 //   RESOLVE_MATCH: '@@found/RESOLVE_MATCH';
 // };
 
-export type Params = Record<string, string>;
+export type Params = Record<string, string | undefined>;
 
 export type ParamsDescriptor = Record<
   string,
-  string | number | boolean | Record<string, unknown>
+  string | number | boolean | Record<string, unknown> | undefined
 >;
 
 // These need to be interfaces to avoid circular reference issues.
diff --git a/test/Matcher.test.ts b/test/Matcher.test.ts
index a887e06c..94c97763 100644
--- a/test/Matcher.test.ts
+++ b/test/Matcher.test.ts
@@ -223,8 +223,7 @@ describe('Matcher', () => {
       const missingMatch = matcher.match({ pathname: '/a' });
 
       if (missingMatch != null && 0 in missingMatch.params) {
-        // FIXME: This type is wrong. It should be string | undefined.
-        const param: string = missingMatch.params[0];
+        const param: string | undefined = missingMatch.params[0];
         expect(param).toBeUndefined();
       }