diff --git a/composer.json b/composer.json
index 2f8dcd0..612a83a 100644
--- a/composer.json
+++ b/composer.json
@@ -29,7 +29,8 @@
         "guzzlehttp/guzzle": "^8.0@dev",
         "guzzlehttp/promises": "^3.0@dev",
         "guzzlehttp/psr7": "^3.0@dev",
-        "psr/http-client": "^1.0"
+        "psr/http-client": "^1.0",
+        "symfony/polyfill-php80": "^1.25"
     },
     "require-dev": {
         "bamarni/composer-bin-plugin": "^1.8.2",
diff --git a/src/ServiceClient.php b/src/ServiceClient.php
index 8e9c7a3..daeb7d8 100644
--- a/src/ServiceClient.php
+++ b/src/ServiceClient.php
@@ -157,8 +157,7 @@ public function executeAllAsync(iterable $commands, array $options = []): Promis
         $promises = function () use ($commands): \Generator {
             foreach ($commands as $key => $command) {
                 if (!$command instanceof CommandInterface) {
-                    throw new \InvalidArgumentException('The iterator must '
-                        .'yield instances of '.CommandInterface::class);
+                    throw new \InvalidArgumentException(\sprintf('The iterator must yield instances of %s; got %s', CommandInterface::class, \get_debug_type($command)));
                 }
                 yield $key => $this->executeAsync($command);
             }
@@ -181,7 +180,7 @@ public function executeAllAsync(iterable $commands, array $options = []): Promis
     public function __call(string $name, array $args)
     {
         $args = isset($args[0]) ? $args[0] : [];
-        if (substr($name, -5) === 'Async') {
+        if (str_ends_with($name, 'Async')) {
             $command = $this->getCommand(substr($name, 0, -5), $args);
 
             return $this->executeAsync($command);
diff --git a/tests/ServiceClientTest.php b/tests/ServiceClientTest.php
index a3a7cc9..a8a0c30 100644
--- a/tests/ServiceClientTest.php
+++ b/tests/ServiceClientTest.php
@@ -274,6 +274,7 @@ public function testMultipleCommandsFailsForNonCommands(): void
         };
 
         $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('got string');
 
         $client = $this->getServiceClient([]);
         $client->executeAll($generateCommands());