diff --git a/netwerk/protocol/http/src/nsHttpChannel.cpp b/netwerk/protocol/http/src/nsHttpChannel.cpp
index 0faace04c02..aad2237c468 100644
--- a/netwerk/protocol/http/src/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/src/nsHttpChannel.cpp
@@ -3431,7 +3431,7 @@ nsHttpChannel::GetContentType(nsACString &value)
 NS_IMETHODIMP
 nsHttpChannel::SetContentType(const nsACString &value)
 {
-    if (mListener) {
+    if (mListener || mWasOpened) {
         if (!mResponseHead)
             return NS_ERROR_NOT_AVAILABLE;
 
diff --git a/netwerk/test/unit/test_bug331825.js b/netwerk/test/unit/test_bug331825.js
index 08b68b1d70e..b852579c935 100644
--- a/netwerk/test/unit/test_bug331825.js
+++ b/netwerk/test/unit/test_bug331825.js
@@ -1,7 +1,7 @@
 do_import_script("netwerk/test/httpserver/httpd.js");
 
 var server;
-var BUGID = "331825";
+const BUGID = "331825";
 
 function TestListener() {
 }
diff --git a/netwerk/test/unit/test_bug369787.js b/netwerk/test/unit/test_bug369787.js
new file mode 100644
index 00000000000..3bec08f5884
--- /dev/null
+++ b/netwerk/test/unit/test_bug369787.js
@@ -0,0 +1,59 @@
+do_import_script("netwerk/test/httpserver/httpd.js");
+
+const BUGID = "369787";
+var server = null;
+var channel = null;
+
+function change_content_type() {
+  var origType = channel.contentType;
+  const newType = "x-foo/x-bar";
+  channel.contentType = newType;
+  do_check_eq(channel.contentType, newType);
+  channel.contentType = origType;
+  do_check_eq(channel.contentType, origType);
+}
+
+function TestListener() {
+}
+TestListener.prototype.onStartRequest = function(request, context) {
+  change_content_type();
+}
+TestListener.prototype.onStopRequest = function(request, context, status) {
+  change_content_type();
+
+  do_timeout(0, "after_channel_closed()");
+}
+
+function after_channel_closed() {
+  try {
+    change_content_type();
+  } finally {
+    server.stop();
+    do_test_finished();
+  }
+}
+
+function run_test() {
+  // start server
+  server = new nsHttpServer();
+
+  server.registerPathHandler("/bug" + BUGID, bug369787);
+
+  server.start(4444);
+
+  // make request
+  channel =
+      Components.classes["@mozilla.org/network/io-service;1"].
+      getService(Components.interfaces.nsIIOService).
+      newChannel("http://localhost:4444/bug" + BUGID, null, null);
+
+  channel.QueryInterface(Components.interfaces.nsIHttpChannel);
+  channel.asyncOpen(new TestListener(), null);
+
+  do_test_pending();
+}
+
+// PATH HANDLER FOR /bug369787
+function bug369787(metadata, response) {
+  /* do nothing */
+}