diff --git a/ambar-core/src/json/decoder.ts b/ambar-core/src/json/decoder.ts
index f19b841..b1477c3 100644
--- a/ambar-core/src/json/decoder.ts
+++ b/ambar-core/src/json/decoder.ts
@@ -358,5 +358,5 @@ const json: Decoder<Json> = rec((json) =>
   oneOf([nullP, string, number, boolean, array(json), objectMap(json)])
 );
 
-const stringEnum = <T extends string[]>(strs: T): Decoder<T[number]> =>
+const stringEnum = <const T extends string[]>(strs: T): Decoder<T[number]> =>
   oneOf(strs.map(stringLiteral));
diff --git a/ambar-core/src/json/encoder.ts b/ambar-core/src/json/encoder.ts
index ed25570..21bdae1 100644
--- a/ambar-core/src/json/encoder.ts
+++ b/ambar-core/src/json/encoder.ts
@@ -164,7 +164,7 @@ const oneOf = <V>(f: (v: V) => Encoder<V>): Encoder<V> =>
     return encoder.run(input);
   });
 
-const stringEnum = <T extends string[]>(strs: T): Encoder<T[number]> =>
+const stringEnum = <const T extends string[]>(strs: T): Encoder<T[number]> =>
   string.rmap((input) => {
     if (!strs.includes(input)) {
       throw new Error(
diff --git a/ambar-core/src/json/schema.ts b/ambar-core/src/json/schema.ts
index 7478ca9..cb65319 100644
--- a/ambar-core/src/json/schema.ts
+++ b/ambar-core/src/json/schema.ts
@@ -177,7 +177,7 @@ const stringLiteral = <T extends string>(str: T): Schema<T> =>
     })
   );
 
-const stringEnum = <T extends string[]>(strs: T): Schema<T[number]> =>
+const stringEnum = <const T extends string[]>(strs: T): Schema<T[number]> =>
   new Schema(D.stringEnum(strs), E.stringEnum(strs));
 
 const oneOf = <V>(f: (v: V) => Schema<V>, ss: Array<Schema<V>>): Schema<V> =>