diff --git a/root-cert-stats/background.js b/root-cert-stats/background.js
index 272fedad..24edf50e 100644
--- a/root-cert-stats/background.js
+++ b/root-cert-stats/background.js
@@ -1,6 +1,9 @@
 "use strict";
 
-let rootCertStats = {};
+// Note: declared with "var" because popup.js references this global variable.
+// If this were to be declared with "const" or "let", then the variable would
+// still be available to this file, but not to popup.js.
+var rootCertStats = {};
 
 /*
 On an onHeadersReceived event, if there was a successful TLS connection