diff --git a/src/Endpoint/Index.php b/src/Endpoint/Index.php
index bd7c210..8f41e63 100644
--- a/src/Endpoint/Index.php
+++ b/src/Endpoint/Index.php
@@ -62,10 +62,6 @@ class Index
         }
 
         if ($filter = $context->getRequest()->getQueryParams()['filter'] ?? null) {
-            if (! is_array($filter)) {
-                throw (new BadRequestException('filter must be an array'))->setSourceParameter('filter');
-            }
-
             $resourceType->applyFilters($query, $filter, $context);
         }
 
diff --git a/src/ResourceType.php b/src/ResourceType.php
index ffe5315..6ddc8f5 100644
--- a/src/ResourceType.php
+++ b/src/ResourceType.php
@@ -110,12 +110,17 @@ final class ResourceType
     /**
      * Apply the resource type's filters to a query.
      */
-    public function applyFilters($query, array $filters, Context $context): void
+    public function applyFilters($query, $filters, Context $context): void
     {
         $schema = $this->getSchema();
         $customFilters = $schema->getFilters();
         $fields = $schema->getFields();
 
+        if (is_string($filters)) {
+            $this->adapter->filterByExpression($query, $filters);
+            return;
+        }
+
         foreach ($filters as $name => $value) {
             if ($name === 'id') {
                 $this->adapter->filterByIds($query, explode(',', $value));