diff --git a/CodeElements/Plc_client/plc_client_os.py b/CodeElements/Plc_client/plc_client_os.py
index 773e77e..a14cce5 100644
--- a/CodeElements/Plc_client/plc_client_os.py
+++ b/CodeElements/Plc_client/plc_client_os.py
@@ -919,6 +919,10 @@ def __get_grid_cell_location(self, x_pos, y_pos):
         self.grid_height = float(self.height/self.grid_row)
         row = math.floor(y_pos / self.grid_height)
         col = math.floor(x_pos / self.grid_width)
+        # Clamp to legal range [0, grid_*-1] — floor() returns grid_*
+        # at the canvas extent, one past the last valid routing index.
+        row = max(0, min(row, self.grid_row - 1))
+        col = max(0, min(col, self.grid_col - 1))
         return row, col
 
     def __get_grid_location_position(self, col:int, row:int):