diff --git a/lib/gui.py b/lib/gui.py
index 705bf02..ddf301e 100644
--- a/lib/gui.py
+++ b/lib/gui.py
@@ -85,7 +85,11 @@ class TreeView():
                     done_items.add(c)
         num_done = len(done_items)
         num_undone = len(undone_items)
-        percentage = round(num_done / (num_done + num_undone) * 100, 1)
+        if num_done + num_undone == 0:
+            percentage = 0
+        else:
+            percentage = round(num_done / (num_done + num_undone) * 100, 1)
+
         heading = "{percent}% ({done} ✔, {undone} ☐)".format(percent=percentage, done=num_done, undone=num_undone) 
         self.tree.heading('#0', text=heading, anchor=tk.W)