diff --git a/types/sha1/index.d.ts b/types/sha1/index.d.ts
index bb55d2f4fc..b65c456054 100644
--- a/types/sha1/index.d.ts
+++ b/types/sha1/index.d.ts
@@ -12,10 +12,19 @@
  * @param options - an options object
  * @returns the resultant SHA1 hash of the given message
  */
+declare function main(message: string | Buffer, options?: Sha1AsStringOptions): string;
+declare function main(message: string | Buffer, options?: Sha1AsBytesOptions): Uint8Array;
 declare function main(message: string | Buffer, options?: Sha1Options): string | Uint8Array;
 export = main;
 
-interface Sha1Options {
-    asBytes?: boolean;
+interface Sha1AsStringOptions {
+    asBytes?: false;
     asString?: boolean;
 }
+
+interface Sha1AsBytesOptions {
+    asBytes: true;
+    asString?: false;
+}
+
+type Sha1Options = Sha1AsStringOptions | Sha1AsBytesOptions;
diff --git a/types/sha1/sha1-tests.ts b/types/sha1/sha1-tests.ts
index 6ce833b10b..12feb5b3d8 100644
--- a/types/sha1/sha1-tests.ts
+++ b/types/sha1/sha1-tests.ts
@@ -9,3 +9,7 @@ const testThree = sha1('message', {asBytes: true});
 fs.readFile('sha1.d.ts', (err: Error, buf: Buffer) => {
     console.log(sha1(buf));
 });
+
+const asHexString: string = sha1('message');
+const asBinString: string = sha1('message', {asString: true});
+const asBytes: Uint8Array = sha1('message', {asBytes: true});