diff --git a/index.d.ts b/index.d.ts
index 7b9e2b16..d21a974f 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -4,7 +4,7 @@ declare module 'https-proxy-agent' {
     namespace HttpsProxyAgent {
         interface HttpsProxyAgentOptions {
             host: string
-            port: number
+            port: number | string
             secureProxy?: boolean
             headers?: {
                 [key: string]: string