diff --git a/RescriptSchema.gen.d.ts b/RescriptSchema.gen.d.ts
index f6e7da23..895041df 100644
--- a/RescriptSchema.gen.d.ts
+++ b/RescriptSchema.gen.d.ts
@@ -25,7 +25,7 @@ export type S_t<Output, Input = unknown> = {
   serialize(value: Output): Result<Input>;
   serializeOrThrow(value: Output): Input;
   serializeToJsonOrThrow(value: Output): Json;
-  assert(data: unknown): asserts data is Output;
+  assert(data: unknown): asserts data is Input;
 };
 
 export abstract class S_Path_t {
diff --git a/docs/js-usage.md b/docs/js-usage.md
index 203b6967..21db2de6 100644
--- a/docs/js-usage.md
+++ b/docs/js-usage.md
@@ -514,7 +514,7 @@ let nodeSchema = S.recursive<Node>((nodeSchema) =>
 ```ts
 const shortStringSchema = S.refine(S.string, (value, s) =>
   if (value.length > 255) {
-    throw s.fail("String can't be more than 255 characters")
+    s.fail("String can't be more than 255 characters")
   }
 )
 ```
@@ -555,7 +555,7 @@ const intToString = (schema) =>
     (string, s) => {
       const int = parseInt(string, 10);
       if (isNaN(int)) {
-        throw s.fail("Can't convert string to int");
+        s.fail("Can't convert string to int");
       }
       return int;
     }
diff --git a/packages/tests/src/core/S_test.ts b/packages/tests/src/core/S_test.ts
index 747f964c..7c040d1a 100644
--- a/packages/tests/src/core/S_test.ts
+++ b/packages/tests/src/core/S_test.ts
@@ -424,7 +424,7 @@ test("Fails to parse with transform with user error", (t) => {
   const schema = S.transform(S.string, (string, s) => {
     const number = Number(string);
     if (Number.isNaN(number)) {
-      throw s.fail("Invalid number");
+      s.fail("Invalid number");
     }
     return number;
   });
diff --git a/src/S.d.ts b/src/S.d.ts
index 0fd167d7..ca90f759 100644
--- a/src/S.d.ts
+++ b/src/S.d.ts
@@ -3,7 +3,7 @@ export { Json, Result, S_error as Error } from "../RescriptSchema.gen";
 
 export type EffectCtx<Output, Input> = {
   schema: Schema<Output, Input>;
-  fail: (message: string) => void;
+  fail: (message: string) => never;
 };
 
 export type Schema<Output, Input = Output> = S_t<Output, Input>;