diff --git a/dom/base/SiteSpecificUserAgent.js b/dom/base/SiteSpecificUserAgent.js
index 288da6cde80..d950b5a8212 100644
--- a/dom/base/SiteSpecificUserAgent.js
+++ b/dom/base/SiteSpecificUserAgent.js
@@ -47,7 +47,7 @@ SiteSpecificUserAgent.prototype = {
     }
 
     let data = { uri: aURI };
-    let result = cpmm.sendSyncMessage("Useragent:GetOverride", data)[0] || HTTP_PROTO_HANDLER.userAgent;
+    let result = cpmm.sendRpcMessage("Useragent:GetOverride", data)[0] || HTTP_PROTO_HANDLER.userAgent;
 
     if (this.userAgentCache.size >= MAX_CACHE_SIZE) {
       this.userAgentCache.clear();
diff --git a/dom/base/test/chrome/cpows_parent.xul b/dom/base/test/chrome/cpows_parent.xul
index bc69541219f..95526b7587b 100644
--- a/dom/base/test/chrome/cpows_parent.xul
+++ b/dom/base/test/chrome/cpows_parent.xul
@@ -66,6 +66,7 @@
       ok(obj.a === 3, "constructor call");
       ok(document.title === "Hello, Kitty", "document node");
       is(typeof document.cookie, "string", "can get document.cookie");
+      is(typeof document.defaultView.navigator.userAgent, "string", "can get navigator.userAgent");
 
       data.i = 6;
       data.b = false;