diff --git a/src/context/json_schema_provider.js b/src/context/json_schema_provider.js
index ffa58446..56f6b4e6 100644
--- a/src/context/json_schema_provider.js
+++ b/src/context/json_schema_provider.js
@@ -48,8 +48,8 @@ class JSONSchemaFormDataProvider extends ContextProvider {
         const schema = this.uiSchema;
         return _.each(schema, (v, k, l) => {
             l[k] = { ...v, ...this.defaultFieldStyles };
-            // retain any prior class attributes
-            l[k].classNames = `${v.classNames} ${this.defaultClassNames}`;
+            l[k].classNames = `${v.classNames || ""} ${this.defaultClassNames || ""}`;
+            return null;
         });
     }
 }