diff --git a/app/assets/javascripts/components/expander.js b/app/assets/javascripts/components/expander.js
index 7a2812131..b7befeb53 100644
--- a/app/assets/javascripts/components/expander.js
+++ b/app/assets/javascripts/components/expander.js
@@ -74,7 +74,7 @@ window.GOVUK.Modules = window.GOVUK.Modules || {}; // if this ; is omitted, none
       try {
         buttonAttributes = JSON.parse(buttonAttributes)
         for (var rawKey in buttonAttributes) {
-          var key = rawKey.replace(/_/i, '-').toLowerCase()
+          var key = rawKey.replace(/_/g, '-').toLowerCase()
           var rawValue = buttonAttributes[rawKey]
           var value = typeof rawValue === 'object' ? JSON.stringify(rawValue) : rawValue
           $button.setAttribute('data-' + key, value)
diff --git a/spec/javascripts/components/expander-spec.js b/spec/javascripts/components/expander-spec.js
index 044e38415..16060d543 100644
--- a/spec/javascripts/components/expander-spec.js
+++ b/spec/javascripts/components/expander-spec.js
@@ -131,7 +131,8 @@ describe('An expander module', function () {
       ga4_expandable: '',
       ga4_event: {
         event_name: 'select_content',
-        type: 'finder'
+        type: 'finder',
+        test_attribute_with_many_underscores: 'oh yes'
       }
     }