diff --git a/src/pyj/dom.pyj b/src/pyj/dom.pyj
index f7bb2388f8..fafbad31a7 100644
--- a/src/pyj/dom.pyj
+++ b/src/pyj/dom.pyj
@@ -103,10 +103,10 @@ def ensure_id(w):
         w.setAttribute('id', ans)
     return ans
 
-extra_css = []
+extra_css = v'[]'
 
 def add_extra_css(func):
-    extra_css.push(func)
+    extra_css.push(func())
 
 def get_widget_css():
     return extra_css.join('\n')