diff --git a/src/Model.php b/src/Model.php
index 2aa8108e2b..2ed67ecad0 100644
--- a/src/Model.php
+++ b/src/Model.php
@@ -577,19 +577,22 @@ public function allFields()
         return $this;
     }
 
-    private function checkOnlyFieldsField(string $field)
+    private function checkOnlyFieldsField(string $fieldName)
     {
-        $this->getField($field); // test if field exists
+        $field = $this->getField($fieldName); // test if field exists
 
-        if ($this->only_fields) {
-            if (!in_array($field, $this->only_fields, true) && !$this->getField($field)->system) {
-                throw (new Exception('Attempt to use field outside of those set by onlyFields'))
-                    ->addMoreInfo('field', $field)
-                    ->addMoreInfo('only_fields', $this->only_fields);
-            }
+        if (!$this->isOnlyField($fieldName) && !$field->system) {
+            throw (new Exception('Attempt to use field outside of those set by onlyFields'))
+                ->addMoreInfo('field', $fieldName)
+                ->addMoreInfo('only_fields', $this->only_fields);
         }
     }
 
+    public function isOnlyField(string $fieldName)
+    {
+        return !$this->only_fields || in_array($fieldName, $this->only_fields, true);
+    }
+
     /**
      * Will return true if specified field is dirty.
      */
@@ -663,11 +666,6 @@ public function getFields($filter = null): array
         }, ARRAY_FILTER_USE_BOTH);
     }
 
-    public function isOnlyField($fieldName)
-    {
-        return !$this->only_fields || in_array($fieldName, $this->only_fields, true);
-    }
-
     /**
      * Set field value.
      *