diff --git a/src/definitions/jsonSchema7.ts b/src/definitions/jsonSchema7.ts
index 86af794..b750876 100644
--- a/src/definitions/jsonSchema7.ts
+++ b/src/definitions/jsonSchema7.ts
@@ -54,7 +54,6 @@ export type JSONSchema7 =
       // Required to allow array values in default field
       // https://github.com/ThomasAribart/json-schema-to-ts/issues/80
       default?: unknown;
-      [key: string]: unknown;
     });
 
 export type JSONSchema7Reference = JSONSchema7 & { $id: string };
diff --git a/src/definitions/jsonSchema7.type.test.ts b/src/definitions/jsonSchema7.type.test.ts
index 4728c51..a8c4cf7 100644
--- a/src/definitions/jsonSchema7.type.test.ts
+++ b/src/definitions/jsonSchema7.type.test.ts
@@ -44,10 +44,3 @@ const schemaWithArrayDefault: JSONSchema = {
   },
 } as const;
 schemaWithArrayDefault;
-
-// Should work with extended schema (custom property)
-const schemaWithCustomProperty: JSONSchema = {
-  type: "object",
-  transform: ["trim"],
-} as const;
-schemaWithCustomProperty;
diff --git a/src/index.ts b/src/index.ts
index b2cf0b2..c1e758b 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -47,5 +47,7 @@ export type JSONSchema = JSONSchema7;
 export type FromSchema<
   S extends JSONSchema,
   Opt extends FromSchemaOptions = FromSchemaDefaultOptions,
-  W extends $JSONSchema7 = Writable<S>,
+  W extends $JSONSchema7 = S extends Record<string | number | symbol, unknown>
+    ? Writable<S>
+    : S,
 > = M.$Resolve<ParseSchema<W, ParseOptions<W, Opt>>>;
diff --git a/src/tests/readme/not.type.test.ts b/src/tests/readme/not.type.test.ts
index b4d09ca..13a7fbe 100644
--- a/src/tests/readme/not.type.test.ts
+++ b/src/tests/readme/not.type.test.ts
@@ -101,6 +101,24 @@ type AssertOddNumber = A.Equals<ReceivedOddNumber, ExpectedOddNumber>;
 const assertOddNumber: AssertOddNumber = 1;
 assertOddNumber;
 
+// Incorrect
+
+const incorrectSchema = {
+  type: "number",
+  not: { bogus: "option" },
+} as const;
+
+type ReceivedIncorrect = FromSchema<
+  // @ts-expect-error
+  typeof incorrectSchema,
+  { parseNotKeyword: true }
+>;
+type ExpectedIncorrect = unknown;
+
+type AssertIncorrect = A.Equals<ReceivedIncorrect, ExpectedIncorrect>;
+const assertIncorrect: AssertIncorrect = 1;
+assertIncorrect;
+
 // Refinment types
 
 const goodLanguageSchema = {