diff --git a/phpstan-baseline.neon b/phpstan-baseline.neon
index 8452437b..09308c64 100644
--- a/phpstan-baseline.neon
+++ b/phpstan-baseline.neon
@@ -2045,16 +2045,6 @@ parameters:
 			count: 1
 			path: src/Component/NestedToken/NestedTokenBuilder.php
 
-		-
-			message: "#^Parameter \\#2 \\$protectedHeader of method Jose\\\\Component\\\\Signature\\\\JWSBuilder\\:\\:addSignature\\(\\) expects array\\{alg\\?\\: string, string\\?\\: mixed\\}, array\\<string, mixed\\> given\\.$#"
-			count: 1
-			path: src/Component/NestedToken/NestedTokenBuilder.php
-
-		-
-			message: "#^Parameter \\#3 \\$header of method Jose\\\\Component\\\\Signature\\\\JWSBuilder\\:\\:addSignature\\(\\) expects array\\{alg\\?\\: string, string\\?\\: mixed\\}, array\\<string, mixed\\> given\\.$#"
-			count: 1
-			path: src/Component/NestedToken/NestedTokenBuilder.php
-
 		-
 			message: "#^Result of \\|\\| is always false\\.$#"
 			count: 2
diff --git a/src/Component/Signature/JWSBuilder.php b/src/Component/Signature/JWSBuilder.php
index b9b2c7a4..84201cc6 100644
--- a/src/Component/Signature/JWSBuilder.php
+++ b/src/Component/Signature/JWSBuilder.php
@@ -19,6 +19,7 @@
 use function count;
 use function in_array;
 use function is_array;
+use function is_string;
 
 class JWSBuilder
 {
@@ -79,8 +80,8 @@ public function withPayload(string $payload, bool $isPayloadDetached = false): s
     /**
      * Adds the information needed to compute the signature. This method will return a new JWSBuilder object.
      *
-     * @param array{alg?: string, string?: mixed} $protectedHeader
-     * @param array{alg?: string, string?: mixed} $header
+     * @param array<string, mixed> $protectedHeader
+     * @param array<string, mixed> $header
      */
     public function addSignature(JWK $signatureKey, array $protectedHeader, array $header = []): self
     {
@@ -185,26 +186,25 @@ private function checkB64AndCriticalHeader(array $protectedHeader): void
     }
 
     /**
-     * @param array{alg?: string, string?: mixed} $protectedHeader
-     * @param array{alg?: string, string?: mixed} $header
+     * @param array<string, mixed> $protectedHeader
+     * @param array<string, mixed> $header
      * @return MacAlgorithm|SignatureAlgorithm
      */
     private function findSignatureAlgorithm(JWK $key, array $protectedHeader, array $header): Algorithm
     {
         $completeHeader = [...$header, ...$protectedHeader];
-        if (! array_key_exists('alg', $completeHeader)) {
+        $alg = $completeHeader['alg'] ?? null;
+        if (! is_string($alg)) {
             throw new InvalidArgumentException('No "alg" parameter set in the header.');
         }
-        if ($key->has('alg') && $key->get('alg') !== $completeHeader['alg']) {
-            throw new InvalidArgumentException(sprintf(
-                'The algorithm "%s" is not allowed with this key.',
-                $completeHeader['alg']
-            ));
+        $keyAlg = $key->has('alg') ? $key->get('alg') : null;
+        if (is_string($keyAlg) && $keyAlg !== $alg) {
+            throw new InvalidArgumentException(sprintf('The algorithm "%s" is not allowed with this key.', $alg));
         }
 
-        $algorithm = $this->signatureAlgorithmManager->get($completeHeader['alg']);
+        $algorithm = $this->signatureAlgorithmManager->get($alg);
         if (! $algorithm instanceof SignatureAlgorithm && ! $algorithm instanceof MacAlgorithm) {
-            throw new InvalidArgumentException(sprintf('The algorithm "%s" is not supported.', $completeHeader['alg']));
+            throw new InvalidArgumentException(sprintf('The algorithm "%s" is not supported.', $alg));
         }
 
         return $algorithm;