diff --git a/index.d.ts b/index.d.ts
index 1d99892..826c3be 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1 +1,4 @@
-export function exec(cmd: string, options: { name?: string, icns?: string, env?: { [key: string]: string } }, callback: (error: string, stdout: string, stderr: string) => void): void;
+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;