diff --git a/src/AccessToken.php b/src/AccessToken.php
index 0afc4ca1e..b7eebb5fa 100644
--- a/src/AccessToken.php
+++ b/src/AccessToken.php
@@ -38,6 +38,7 @@
 use SimpleJWT\JWT as SimpleJWT;
 use SimpleJWT\Keys\KeyFactory;
 use SimpleJWT\Keys\KeySet;
+use TypeError;
 use UnexpectedValueException;
 
 /**
@@ -399,6 +400,10 @@ private function checkAndInitializePhpsec()
         }
     }
 
+    /**
+     * @return string
+     * @throws TypeError If the key cannot be initialized to a string.
+     */
     private function loadPhpsecPublicKey(string $modulus, string $exponent): string
     {
         if (class_exists(RSA::class) && class_exists(BigInteger2::class)) {
@@ -421,7 +426,11 @@ private function loadPhpsecPublicKey(string $modulus, string $exponent): string
                 $exponent
             ]), 256),
         ]);
-        return $key->toString('PKCS8');
+        $formattedPublicKey = $key->toString('PKCS8');
+        if (!is_string($formattedPublicKey)) {
+            throw new TypeError('Failed to initialize the key');
+        }
+        return $formattedPublicKey;
     }
 
     /**