diff --git a/src/Form/FieldInput.js b/src/Form/FieldInput.js
index <HASH>..<HASH> 100644
--- a/src/Form/FieldInput.js
+++ b/src/Form/FieldInput.js
@@ -107,7 +107,11 @@ class FieldInput extends Util.mixin(BindMixin) {
       return null;
     }
 
-    return <span className={helpBlockClass}>{helpBlock}</span>;
+    return (
+      <span className={classNames(helpBlockClass)}>
+        {helpBlock}
+      </span>
+    );
   }
 
   getLabel() {
Passing class through classNames function