Learned that I should not refer to env vars directly so I removed all references.

app/Transformers/CategoryTransformer.php

@@ -31,6 +31,7 @@ use FireflyIII\Models\TransactionCurrency;
 use FireflyIII\Repositories\Category\CategoryRepositoryInterface;
 use Illuminate\Support\Collection;
 use League\Fractal\TransformerAbstract;
+use Log;
 use Symfony\Component\HttpFoundation\ParameterBag;
 
 /**
@@ -51,6 +52,9 @@ class CategoryTransformer extends TransformerAbstract
     public function __construct(ParameterBag $parameters)
     {
         $this->parameters = $parameters;
+        if ('testing' === config('app.env')) {
+            Log::warning(sprintf('%s should not be instantiated in the TEST environment!', \get_class($this)));
+        }
     }
 
     /**