diff --git a/src/PixivTypes.ts b/src/PixivTypes.ts
index fc8315e..3f204f7 100644
--- a/src/PixivTypes.ts
+++ b/src/PixivTypes.ts
@@ -76,7 +76,7 @@ export interface PixivParams {
 }
 
 export interface PixivFetchOptions {
-  data?: string
+  data?: PixivParams
   method?: string
   headers?: {
     [header: string]: string
diff --git a/src/index.ts b/src/index.ts
index 677a55d..b2c8974 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -681,7 +681,7 @@ export default class PixivApp<CamelcaseKeys extends boolean = true> {
       options.headers = {
         'Content-Type': 'application/x-www-form-urlencoded',
       }
-      options.data = stringify(decamelizeKeys(options.data))
+      options.data = stringify(decamelizeKeys(options.data)) as PixivParams
     }
 
     if (options.params) {