diff --git a/src/Signer/Rsa.php b/src/Signer/Rsa.php
index a89ff456..0db19c57 100644
--- a/src/Signer/Rsa.php
+++ b/src/Signer/Rsa.php
@@ -26,7 +26,12 @@ public function createHash($payload, Key $key)
         $this->validateKey($key);
 
         $signature = '';
-        openssl_sign($payload, $signature, $key, $this->getAlgorithm());
+
+        if (!openssl_sign($payload, $signature, $key, $this->getAlgorithm())) {
+            throw new InvalidArgumentException(
+                'There was an error while creating the signature: ' . openssl_error_string()
+            );
+        }
 
         return $signature;
     }
@@ -47,8 +52,6 @@ public function doVerify($expected, $payload, Key $key)
      *
      * @param resource $key
      *
-     * @return boolean
-     *
      * @throws InvalidArgumentException
      */
     private function validateKey($key)