diff --git a/src/Tool/Command/Assert.php b/src/Tool/Command/Assert.php
index 796f3527..b1a4f5d9 100644
--- a/src/Tool/Command/Assert.php
+++ b/src/Tool/Command/Assert.php
@@ -11,7 +11,7 @@ public static function requireFields(array $fields, array $data, string $type)
         });
 
         if (!empty($missingFields)) {
-            throw new \InvalidArgumentException(\sprintf('Missing fields "%s" in the %s: `%s`.', \implode($missingFields, ', '), $type, \json_encode($data)));
+            throw new \InvalidArgumentException(\sprintf('Missing fields "%s" in the %s: `%s`.', \implode(', ', $missingFields), $type, \json_encode($data)));
         }
     }
 }