diff --git a/types/index.d.ts b/types/index.d.ts
index 744e92f..d951c4b 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -14,7 +14,7 @@ declare namespace anchor {
   }
 
   export interface HeaderLinkPermalinkOptions extends PermalinkOptions {
-    safariReaderFix: boolean;
+    safariReaderFix?: boolean;
   }
 
   export interface LinkAfterHeaderPermalinkOptions extends PermalinkOptions {
@@ -58,10 +58,10 @@ declare namespace anchor {
   }
 
   export const permalink: {
-    headerLink: (opts: HeaderLinkPermalinkOptions) => PermalinkGenerator
-    linkAfterHeader: (opts: LinkAfterHeaderPermalinkOptions) => PermalinkGenerator
-    linkInsideHeader: (opts: LinkInsideHeaderPermalinkOptions) => PermalinkGenerator
-    ariaHidden: (opts: AriaHiddenPermalinkOptions) => PermalinkGenerator
+    headerLink: (opts?: HeaderLinkPermalinkOptions) => PermalinkGenerator
+    linkAfterHeader: (opts?: LinkAfterHeaderPermalinkOptions) => PermalinkGenerator
+    linkInsideHeader: (opts?: LinkInsideHeaderPermalinkOptions) => PermalinkGenerator
+    ariaHidden: (opts?: AriaHiddenPermalinkOptions) => PermalinkGenerator
   };
 }