diff --git a/shifts.py b/shifts.py
index e84e93b..0608c51 100644
--- a/shifts.py
+++ b/shifts.py
@@ -651,9 +651,9 @@ class RotaBuilder(object):
                                      worker.fte_adj)
 
                     if self.use_previous_shifts:
-                        if shift in worker.previous_shifts:
-                            worked, allocated = worker.previous_shifts[shift]
-                            target_shifts = target_shifts + allocated - worked
+                        if shift.name in worker.previous_shifts:
+                            worked, allocated = worker.previous_shifts[shift.name]
+                            target_shifts = target_shifts + float(allocated) - float(worked)
 
 
                     worker.shift_target_number[shift.name] = target_shifts