diff --git a/tiramisu/value.py b/tiramisu/value.py
index 282c996..2ac7add 100644
--- a/tiramisu/value.py
+++ b/tiramisu/value.py
@@ -69,6 +69,8 @@ class Values(object):
     def set_previous_value(self, opt):
         if opt in self.values:
             old_value = self.values[opt]
+        elif opt.is_multi():
+            old_value = []
         else:
             old_value = None
         if type(old_value) == Multi: