diff --git a/README.md b/README.md
index 8709299..943c3ad 100644
--- a/README.md
+++ b/README.md
@@ -64,7 +64,7 @@ Use this if you want to override the default error message of this library.
 // signature
 type errorMessage = (
   methodName: 'assert' | 'debug' | 'error' | 'info' | 'log' | 'warn',
-  bold: (string) => string
+  bold: (string: string) => string
 ) => string
 ```
 
diff --git a/index.d.ts b/index.d.ts
index 89040fe..5fd8c04 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -7,7 +7,7 @@ declare namespace init {
      */
     errorMessage?: (
       methodName: 'assert' | 'debug' | 'error' | 'info' | 'log' | 'warn',
-      bold: (string) => string
+      bold: (string: string) => string
     ) => string
 
     /** @default false */