diff --git a/xlcalculator/model.py b/xlcalculator/model.py
index a62c09b..62ad2c2 100644
--- a/xlcalculator/model.py
+++ b/xlcalculator/model.py
@@ -29,6 +29,7 @@ def set_cell_value(self, address, value):
         if isinstance(address, str):
             if address in self.cells:
                 self.cells[address].value = copy.copy(value)
+                self.cells[address].formula = None
             else:
                 self.cells[address] = xltypes.XLCell(address, copy.copy(value))