diff --git a/src/js/components/utils/Formsy.js b/src/js/components/utils/Formsy.js
index d0f896d5..b4fac027 100644
--- a/src/js/components/utils/Formsy.js
+++ b/src/js/components/utils/Formsy.js
@@ -4,8 +4,12 @@ import Formsy from 'formsy-react';
  * Custom validation rules used throughout the app.
  */
 export default function initFormsy () {
-  Formsy.addValidationRule('isJson', (values, value) => {
-    try { return !!JSON.parse(value); }
+  /*
+   * The field is valid if the current value is either empty
+   * or a valid JSON string.
+   */
+  Formsy.addValidationRule('isJson', (values, value, otherField) => {
+    try { return value === '' ? true : !!JSON.parse(value); }
     catch(e) { return false; }
   });
 }