diff --git a/src/main/java/io/jenkins/plugins/restlistparam/RestListParameterDefinition.java b/src/main/java/io/jenkins/plugins/restlistparam/RestListParameterDefinition.java
index 98f2c7c..27be896 100644
--- a/src/main/java/io/jenkins/plugins/restlistparam/RestListParameterDefinition.java
+++ b/src/main/java/io/jenkins/plugins/restlistparam/RestListParameterDefinition.java
@@ -393,7 +393,7 @@ public FormValidation doTestConfiguration(@AncestorInPath final Item context,
         mimeType,
         0,
         valueExpression,
-        displayExpression,
+        StringUtils.isNotBlank(displayExpression) ? displayExpression : "$",
         filter,
         valueOrder);