diff --git a/ttkwidgets/timeline.py b/ttkwidgets/timeline.py
index 659478d..27e96fa 100644
--- a/ttkwidgets/timeline.py
+++ b/ttkwidgets/timeline.py
@@ -575,7 +575,7 @@ def delete_marker(self, iid):
         :type iid: str
         """
         if iid == tk.ALL:
-            for iid in self.markers.keys():
+            for iid in list(self.markers.keys()):
                 self.delete_marker(iid)
             return
         options = self._markers[iid]