diff --git a/index.d.ts b/index.d.ts
index 826c3be..329c122 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,4 +1,4 @@
 export function exec(cmd: string,
         options?: ((error?: Error, stdout?: string | Buffer, stderr?: string | Buffer) => void)
                 | { name?: string, icns?: string, env?: { [key: string]: string } },
-        callback?: (error: Error, stdout: string | Buffer, stderr: string | Buffer) => void): void;
+        callback?: (error?: Error, stdout?: string | Buffer, stderr?: string | Buffer) => void): void;