diff --git a/.gitignore b/.gitignore
index 3c3629e..a7880cf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,3 @@
 node_modules
+pull_request.md
+PULL_REQUEST.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index cd4dff6..7024695 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ All notable changes to this project will be documented in this file.
 ### Added
 
 - **Extended test coverage**: 8 new tests (60 total) — `{ type: undefined/null/"string" }` schema rejection, `Union(Array, String)` value discrimination, scalar-only coerce tests, and `Union(Object, String)` skipping object-key validation for string values.
+- **Readonly enum array support**: Updated `FieldConfig.enum` to allow `readonly any[]` for `as const` schema declarations.
 
 ### Changed
 
diff --git a/README.md b/README.md
index dc79f72..ea94920 100644
--- a/README.md
+++ b/README.md
@@ -475,13 +475,13 @@ ModelCore throws typed error classes for every failure mode:
 | Error class | Trigger |
 |---|---|
 | `ValidationError` | Custom `validate` hook throws |
-| `TypeValidationError` | Value constructor doesn't match schema type |
+| `TypeValidationError` | Value constructor doesn't match schema type, or value has no prototype chain |
 | `RequiredError` | Required field is missing |
 | `EnumValueError` | Value not in allowed enum set |
 | `RangeError` | Value exceeds min/max |
 | `ImmutableObjectError` | Write to immutable class |
 | `ImmutablePropertyError` | Write to immutable field |
-| `SchemaDefinitionError` | Malformed schema definition |
+| `SchemaDefinitionError` | Malformed schema definition, `{ type: undefined }`, `{ type: null }`, or missing `static schema` |
 | `MissingPropertyError` | Nested required property missing |
 | `ValueError` | Array method misuse (e.g., `fill()`) |
 
@@ -489,6 +489,21 @@ All errors extend `ModelCoreError`, which carries `source`, `path`, `expected`,
 
 ---
 
+## Defensive guarantees
+
+ModelCore guards against several degenerate inputs that would otherwise cause raw `TypeError` crashes or silent data corruption:
+
+| Input | Before | After |
+|---|---|---|
+| `{ type: undefined }` in schema | Raw `TypeError: Cannot read properties of undefined` | `SchemaDefinitionError` with path and code |
+| `{ type: null }` in schema | Raw `TypeError` | `SchemaDefinitionError` with path and code |
+| `Object.create(null)` as field value | Raw `TypeError: Cannot read properties of undefined (reading 'name')` | `TypeValidationError` with message `"...got null-prototype object"` |
+| `Union(Array, String)` with string input | String silently split into char array `['h','e','l','l','o']` | String preserved as-is |
+| `"42"` for a `Number` field (no `coerce: true`) | Silently accepted as string | `TypeValidationError` — use `coerce: true` to auto-convert |
+| Subclass with no `static schema` | Silent empty object | `SchemaDefinitionError` with class name |
+
+
+
 ## Performance
 
 ModelCore achieves **>200K ops/sec** on full model construction with nested containers (5 fields, 100K iterations, Node 24). Scalar-only models reach **~1.9M ops/sec**.
diff --git a/src/typing.d.ts b/src/typing.d.ts
index a99512a..aff1dd9 100644
--- a/src/typing.d.ts
+++ b/src/typing.d.ts
@@ -4,7 +4,7 @@ export interface FieldConfig {
     optional?: boolean;
     required?: boolean;
     default?: any;
-    enum?: any[];
+    enum?: any[] | readonly any[];
     max?: number;
     min?: number;
     beforeChecks?: (value: any) => any;
diff --git a/test/base.test.js b/test/base.test.js
index 5a46dfc..4b41fcb 100644
--- a/test/base.test.js
+++ b/test/base.test.js
@@ -837,6 +837,8 @@ test("error.expected contains meaningful values (type, min, max, enum)", () => {
   }
 });
 
+// ==================== NULL & EDGE-CASE GUARDS ====================
+
 test("schema field with { type: undefined } throws SchemaDefinitionError", () => {
   class U extends Base {
     static schema = { name: { type: undefined } };
@@ -950,3 +952,30 @@ test("Union(Object, String) with string value preserves string (does not run Obj
   assert.equal(typeof u.val, "string");
   assert.equal(u.val, "hello");
 });
+
+test("supports readonly enum arrays (as const assertions)", () => {
+  const RO_ROLES = Object.freeze(["ADMIN", "USER", "GUEST"]);
+  class U extends Base {
+    static schema = {
+      role: { type: String, enum: RO_ROLES }
+    };
+  }
+
+  const u = new U({ role: "ADMIN" });
+  assert.equal(u.role, "ADMIN");
+  assert.throws(() => new U({ role: "INVALID" }), /Invalid value for 'role'/);
+});
+
+test("handles null-prototype inputs gracefully", () => {
+  class U extends Base {
+    static schema = {
+      name: { type: String }
+    };
+  }
+
+  const nullProtoObj = Object.create(null);
+  nullProtoObj.name = "Alice";
+
+  const u = new U(nullProtoObj);
+  assert.equal(u.name, "Alice");
+});