diff --git a/lib/classes/JsonApi/JsonApiIntegration/FieldsetFilter.php b/lib/classes/JsonApi/JsonApiIntegration/FieldsetFilter.php
index 42a5842edbe5eb11d5a7d98d259907957b6ec87f..45dc29d00dc7c490c4377af188b6ec3982f63160 100644
--- a/lib/classes/JsonApi/JsonApiIntegration/FieldsetFilter.php
+++ b/lib/classes/JsonApi/JsonApiIntegration/FieldsetFilter.php
@@ -36,7 +36,7 @@ class FieldsetFilter extends \Neomerx\JsonApi\Representation\FieldSetFilter
      */
     private function resolveValue($value)
     {
-        if (is_callable($value)) {
+        if (!is_string($value) && is_callable($value)) {
             return $value();
         }