diff --git a/src/Client.php b/src/Client.php
index ac96104..1f77e18 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -108,9 +108,9 @@ public static function setHttp(HttpInterface $http): void
     /**
      * Set the user agent reported with API calls.
      *
-     * @param string|null $userAgent
+     * @param string $userAgent
      */
-    public static function setUserAgent(?string $userAgent): void
+    public static function setUserAgent(string $userAgent): void
     {
         static::$userAgent = $userAgent;
     }
@@ -132,7 +132,7 @@ private static function addAuthenticationOptions(array $options): array
 
     private static function addUserAgentOptions(array $options): array
     {
-        if (static::$userAgent === null) {
+        if (static::$userAgent === null || strlen(static::$userAgent) === 0) {
             return $options;
         }
 
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index 5b4bf7a..c00501b 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -20,7 +20,7 @@ public function setup(): void
 
     public function tearDown(): void
     {
-        Client::setUserAgent(null);
+        Client::setUserAgent('');
     }
 
     public function testAuthenticateWithCredentials()