diff --git a/src/lib/options.ts b/src/lib/options.ts
index 2935d2c..41a28c2 100644
--- a/src/lib/options.ts
+++ b/src/lib/options.ts
@@ -14,7 +14,7 @@ type ParsedOptions<P extends Options, V extends string | TextPiece> = {
   error?: string;
 };
 
-export function getOpt<P extends Options>(
+export function getOpt<const P extends Options>(
   argv: TextPiece[],
   options: P,
   procBody: (
@@ -60,7 +60,7 @@ export function getOpt<P extends Options>(
   }
 }
 
-export function getOptRaw<P extends Options>(
+export function getOptRaw<const P extends Options>(
   argv: TextPiece[],
   options: P,
   procBody: (
@@ -95,7 +95,7 @@ export function getOptRaw<P extends Options>(
 
 const SWITCH_REGEX = /^-([^]*)/;
 
-function getOptCore<P extends Options>(
+function getOptCore<const P extends Options>(
   argv: TextPiece[],
   options: P
 ): [ParsedOptions<P, TextPiece>, TextPiece[]] {