diff --git a/Libraries/LibWeb/Crypto/SubtleCrypto.cpp b/Libraries/LibWeb/Crypto/SubtleCrypto.cpp
index 25fd18a8556..f4fedd7e696 100644
--- a/Libraries/LibWeb/Crypto/SubtleCrypto.cpp
+++ b/Libraries/LibWeb/Crypto/SubtleCrypto.cpp
@@ -598,7 +598,7 @@ JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> SubtleCrypto::verify(AlgorithmId
 }
 
 // https://w3c.github.io/webcrypto/#SubtleCrypto-method-deriveBits
-JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> SubtleCrypto::derive_bits(AlgorithmIdentifier algorithm, GC::Ref<CryptoKey> base_key, u32 length)
+JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> SubtleCrypto::derive_bits(AlgorithmIdentifier algorithm, GC::Ref<CryptoKey> base_key, Optional<u32> length_optional)
 {
     auto& realm = this->realm();
     // 1. Let algorithm, baseKey and length, be the algorithm, baseKey and length parameters passed to the deriveBits() method, respectively.
@@ -614,7 +614,7 @@ JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> SubtleCrypto::derive_bits(Algori
     auto promise = WebIDL::create_promise(realm);
 
     // 5. Return promise and perform the remaining steps in parallel.
-    Platform::EventLoopPlugin::the().deferred_invoke(GC::create_function(realm.heap(), [&realm, normalized_algorithm = normalized_algorithm.release_value(), promise, base_key, length]() -> void {
+    Platform::EventLoopPlugin::the().deferred_invoke(GC::create_function(realm.heap(), [&realm, normalized_algorithm = normalized_algorithm.release_value(), promise, base_key, length_optional]() -> void {
         HTML::TemporaryExecutionContext context(realm, HTML::TemporaryExecutionContext::CallbacksEnabled::Yes);
         // 6. If the following steps or referenced procedures say to throw an error, reject promise with the returned error and then terminate the algorithm.
 
@@ -631,7 +631,7 @@ JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> SubtleCrypto::derive_bits(Algori
         }
 
         // 9. Let result be the result of creating an ArrayBuffer containing the result of performing the derive bits operation specified by normalizedAlgorithm using baseKey, algorithm and length.
-        auto result = normalized_algorithm.methods->derive_bits(*normalized_algorithm.parameter, base_key, length);
+        auto result = normalized_algorithm.methods->derive_bits(*normalized_algorithm.parameter, base_key, length_optional);
         if (result.is_error()) {
             WebIDL::reject_promise(realm, promise, Bindings::exception_to_throw_completion(realm.vm(), result.release_error()).release_value().value());
             return;
diff --git a/Libraries/LibWeb/Crypto/SubtleCrypto.h b/Libraries/LibWeb/Crypto/SubtleCrypto.h
index 014e9b478d2..054187d847f 100644
--- a/Libraries/LibWeb/Crypto/SubtleCrypto.h
+++ b/Libraries/LibWeb/Crypto/SubtleCrypto.h
@@ -34,7 +34,7 @@ public:
     GC::Ref<WebIDL::Promise> digest(AlgorithmIdentifier const& algorithm, GC::Root<WebIDL::BufferSource> const& data);
 
     JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> generate_key(AlgorithmIdentifier algorithm, bool extractable, Vector<Bindings::KeyUsage> key_usages);
-    JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> derive_bits(AlgorithmIdentifier algorithm, GC::Ref<CryptoKey> base_key, u32 length);
+    JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> derive_bits(AlgorithmIdentifier algorithm, GC::Ref<CryptoKey> base_key, Optional<u32> length_optional);
     JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> derive_key(AlgorithmIdentifier algorithm, GC::Ref<CryptoKey> base_key, AlgorithmIdentifier derived_key_type, bool extractable, Vector<Bindings::KeyUsage> key_usages);
 
     JS::ThrowCompletionOr<GC::Ref<WebIDL::Promise>> import_key(Bindings::KeyFormat format, KeyDataType key_data, AlgorithmIdentifier algorithm, bool extractable, Vector<Bindings::KeyUsage> key_usages);
diff --git a/Libraries/LibWeb/Crypto/SubtleCrypto.idl b/Libraries/LibWeb/Crypto/SubtleCrypto.idl
index bf47ba840d1..18b141e002b 100644
--- a/Libraries/LibWeb/Crypto/SubtleCrypto.idl
+++ b/Libraries/LibWeb/Crypto/SubtleCrypto.idl
@@ -53,7 +53,8 @@ interface SubtleCrypto {
 
     Promise<any> generateKey(AlgorithmIdentifier algorithm, boolean extractable, sequence<KeyUsage> keyUsages);
     Promise<any> deriveKey(AlgorithmIdentifier algorithm, CryptoKey baseKey, AlgorithmIdentifier derivedKeyType, boolean extractable, sequence<KeyUsage> keyUsages);
-    Promise<ArrayBuffer> deriveBits(AlgorithmIdentifier algorithm, CryptoKey baseKey, unsigned long length);
+    // FIXME: Promise<ArrayBuffer> deriveBits(AlgorithmIdentifier algorithm, CryptoKey baseKey, optional unsigned long? length = null);
+    Promise<ArrayBuffer> deriveBits(AlgorithmIdentifier algorithm, CryptoKey baseKey, optional unsigned long? length);
 
     Promise<CryptoKey> importKey(KeyFormat format, (BufferSource or JsonWebKey) keyData,  AlgorithmIdentifier algorithm, boolean extractable, sequence<KeyUsage> keyUsages);
     Promise<any> exportKey(KeyFormat format, CryptoKey key);