diff --git a/HttpClientTrait.php b/HttpClientTrait.php
index 7cb82c7..2486f04 100644
--- a/HttpClientTrait.php
+++ b/HttpClientTrait.php
@@ -110,7 +110,7 @@ private static function prepareRequest(?string $method, ?string $url, array $opt
             throw new InvalidArgumentException(sprintf('Option "auth_basic" must be string or an array, %s given.', \gettype($options['auth_basic'])));
         }
 
-        if (isset($options['auth_bearer']) && (!\is_string($options['auth_bearer']) || !preg_match('{^[-._~+/0-9a-zA-Z]++=*+$}', $options['auth_bearer']))) {
+        if (isset($options['auth_bearer']) && (!\is_string($options['auth_bearer']) || !preg_match('{^[-._=~+/0-9a-zA-Z]++$}', $options['auth_bearer']))) {
             throw new InvalidArgumentException(sprintf('Option "auth_bearer" must be a string containing only characters from the base 64 alphabet, %s given.', \is_string($options['auth_bearer']) ? 'invalid string' : \gettype($options['auth_bearer'])));
         }