diff --git a/cellpose/gui/gui.py b/cellpose/gui/gui.py
index 4d92732a..1f72131e 100644
--- a/cellpose/gui/gui.py
+++ b/cellpose/gui/gui.py
@@ -1445,6 +1445,8 @@ def done_remove_multiple_cells(self):
             self.remove_roi(self.remove_roi_obj)
 
     def merge_cells(self, idx):
+        if self.selected == 0:
+            return
         self.prev_selected = self.selected
         self.selected = idx
         if self.selected != self.prev_selected: