diff -r 66c7a0826bd7 -r a11c8dba822e src/ldt/ldt/ldt_utils/views/json.py
--- a/src/ldt/ldt/ldt_utils/views/json.py	Tue Jan 22 11:25:55 2013 +0100
+++ b/src/ldt/ldt/ldt_utils/views/json.py	Tue Jan 22 12:44:42 2013 +0100
@@ -178,6 +178,8 @@
                     cur_out = cur_in + dur
                     if tc_in<=cur_in and cur_out<=tc_out and ((not remove_zero_dur) or (remove_zero_dur and dur>0.0)):
                         mashup_list["items"].append(res["element_id"])
+            if mashup_dict["lists"] is None:
+                mashup_dict["lists"] = []
             mashup_dict["lists"].append(mashup_list)
     
             # If asked, we remove the annotations not used in the mashup.