diff --git a/lib/index.js b/lib/index.js
index bab2d2e..4b2c078 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -30,6 +30,12 @@ var GTM = module.exports = integration('Google Tag Manager')
  */
 
 GTM.prototype.initialize = function() {
+  // If window.dataLayer already exists and isn't an array, fail gracefully.
+  if (window.dataLayer && !Array.isArray(window.dataLater)) {
+    this.debug('window.dataLayer already exists - not running Segment<>GTM destination');
+    return;
+  }
+  
   push({ 'gtm.start': Number(new Date()), event: 'gtm.js' });
 
   if (this.options.environment.length) {