diff --git a/contrib/static/nntpchan.js b/contrib/static/nntpchan.js
index 7db7171..5137920 100644
--- a/contrib/static/nntpchan.js
+++ b/contrib/static/nntpchan.js
@@ -36,13 +36,14 @@ function get_storage() {
 }
 
 function enable_theme(prefix, name) {
-
-  var theme = document.getElementById("current_theme");
-  if (theme) {
-    theme.href = prefix + "static/"+ name + ".css";
-    var st = get_storage();
-    st.nntpchan_prefix = prefix;
-    st.nntpchan_theme = name;
+  if (prefix && name) {
+    var theme = document.getElementById("current_theme");
+    if (theme) {
+      theme.href = prefix + "static/"+ name + ".css";
+      var st = get_storage();
+      st.nntpchan_prefix = prefix;
+      st.nntpchan_theme = name;
+    }
   }
 }