diff --git a/types/evaporate/index.d.ts b/types/evaporate/index.d.ts
index 92f02c9c73..8e959f6b64 100644
--- a/types/evaporate/index.d.ts
+++ b/types/evaporate/index.d.ts
@@ -52,7 +52,7 @@ declare namespace Evaporate {
         customAuthMethod?: null | ((
             signParams: string,
             signHeaders: string,
-            stringToSign: () => string | undefined,
+            stringToSign: string,
             signatureDateTime: string,
             canonicalRequest: string
         ) => Promise<string>);