diff --git a/pudb/debugger.py b/pudb/debugger.py
index 2259d824..01f19bb3 100644
--- a/pudb/debugger.py
+++ b/pudb/debugger.py
@@ -1123,7 +1123,9 @@ def edit_inspector_detail(w, size, key):
                     iinfo.access_level = "all"
 
                 if var.watch_expr is not None:
-                    var.watch_expr.expression = watch_edit.get_edit_text()
+                    new_expression = watch_edit.get_edit_text()
+                    if new_expression != var.watch_expr.expression:
+                        var.watch_expr.set_expression(new_expression)
 
             elif result == "del":
                 try:
diff --git a/pudb/var_view.py b/pudb/var_view.py
index 36ebecc7..623c0889 100644
--- a/pudb/var_view.py
+++ b/pudb/var_view.py
@@ -215,6 +215,10 @@ def eval(self, frame_globals, frame_locals):
                 return WatchEvalError()
         return self._value
 
+    def set_expression(self, expression):
+        self.expression = expression
+        self._value = self.NOT_EVALUATED
+
 
 class WatchEvalError:
     def __str__(self):