diff --git a/src/KeySet.php b/src/KeySet.php
index c739fb7..efaf0c5 100644
--- a/src/KeySet.php
+++ b/src/KeySet.php
@@ -13,7 +13,7 @@
  * @see    https://github.com/Strobotti/php-jwk
  * @since 1.0.0
  */
-class KeySet implements \JsonSerializable
+class KeySet implements \JsonSerializable, \Countable, \IteratorAggregate
 {
     /**
      * @var KeyFactory
@@ -84,7 +84,7 @@ public function getKeyById(string $kid, string $use = KeyInterface::PUBLIC_KEY_U
      */
     public function addKey(KeyInterface $key): self
     {
-        if ($this->containsKey($key->getKeyId(), $key->getPublicKeyUse())) {
+        if ($key->getKeyId() && $this->containsKey($key->getKeyId(), $key->getPublicKeyUse())) {
             throw new \InvalidArgumentException(\sprintf('Key with id `%s` and use `%s` already exists in the set', $key->getKeyId(), $key->getPublicKeyUse()));
         }
 
@@ -116,4 +116,20 @@ public function jsonSerialize(): array
             'keys' => \array_values($ret),
         ];
     }
+
+    /**
+     * @since 1.3.0
+     */
+    public function count(): int
+    {
+        return \count($this->keys);
+    }
+
+    /**
+     * @since 1.3.0
+     */
+    public function getIterator(): \ArrayIterator
+    {
+        return new \ArrayIterator($this->keys);
+    }
 }
diff --git a/tests/KeySetTest.php b/tests/KeySetTest.php
index 2130c1c..2729ed5 100644
--- a/tests/KeySetTest.php
+++ b/tests/KeySetTest.php
@@ -97,4 +97,42 @@ public function testGetKeyById(): void
 
         static::assertNull($keySet->getKeyById('asdf'));
     }
+
+    public function testCountable(): void
+    {
+        $keyset = new KeySet();
+        static::assertCount(0, $keyset);
+
+        $keyset->addKey(new Rsa());
+        static::assertCount(1, $keyset);
+
+        $keyset->addKey(new Rsa());
+        static::assertCount(2, $keyset);
+    }
+
+    public function testIteratorAggregate(): void
+    {
+        $keyset = new KeySet();
+
+        $count = 0;
+
+        foreach ($keyset as $key) {
+            ++$count;
+        }
+
+        static::assertSame(0, $count);
+
+        $keyset->addKey(new Rsa());
+        $keyset->addKey(new Rsa());
+        $keyset->addKey(new Rsa());
+
+        foreach ($keyset as $index => $key) {
+            static::assertInstanceOf(Rsa::class, $key);
+            static::assertSame($index, $count);
+
+            ++$count;
+        }
+
+        static::assertSame(3, $count);
+    }
 }