diff --git a/src/decorator/common/IsIn.spec.ts b/src/decorator/common/IsIn.spec.ts
new file mode 100644
index 0000000000..c9027eecc5
--- /dev/null
+++ b/src/decorator/common/IsIn.spec.ts
@@ -0,0 +1,17 @@
+import { isIn } from './IsIn';
+
+describe('@IsIn decorator implementation', () => {
+  describe('isIn validator', () => {
+    it('should accept valid values', () => {
+      expect(isIn('A', ['A', 'B'])).toBe(true);
+      expect(isIn('A', ['B', 'C'])).toBe(false);
+      expect(isIn('A', [1, 2])).toBe(false);
+    });
+
+    it('should not accept invalid values', () => {
+      expect(isIn('A', 5 as any)).toBe(false);
+      expect(isIn('A', 'ABC' as any)).toBe(false);
+      expect(isIn('A', false as any)).toBe(false);
+    });
+  });
+});
diff --git a/src/decorator/common/IsIn.ts b/src/decorator/common/IsIn.ts
index cd9499e5e8..d074dcca5e 100644
--- a/src/decorator/common/IsIn.ts
+++ b/src/decorator/common/IsIn.ts
@@ -7,7 +7,7 @@ export const IS_IN = 'isIn';
  * Checks if given value is in a array of allowed values.
  */
 export function isIn(value: unknown, possibleValues: readonly unknown[]): boolean {
-  return !Array.isArray(possibleValues) || possibleValues.some(possibleValue => possibleValue === value);
+  return Array.isArray(possibleValues) && possibleValues.some(possibleValue => possibleValue === value);
 }
 
 /**