diff --git a/src/calibre/gui2/search_box.py b/src/calibre/gui2/search_box.py
index e307a33430..7e03a0a88d 100644
--- a/src/calibre/gui2/search_box.py
+++ b/src/calibre/gui2/search_box.py
@@ -58,9 +58,9 @@ class SearchLineEdit(QLineEdit):  # {{{
             menu.addAction(ac)
         menu.addSeparator()
         ac = menu.addAction(_('Invert current search'))
-        ac.setEnabled(bool(self.text()))
+        ac.setEnabled(bool(self.text().strip()))
         ac.setIcon(QIcon.ic('search.png'))
-        ac.triggered.connect(self.reverse_search)
+        ac.triggered.connect(self.invert_search)
         menu.addAction(ac)
         menu.addSeparator()
         if self.as_url is not None:
@@ -70,10 +70,16 @@ class SearchLineEdit(QLineEdit):  # {{{
         menu.addAction(_('&Clear search history')).triggered.connect(self.clear_history)
         menu.exec(ev.globalPos())
 
-    def reverse_search(self):
-        self.setText(f'NOT ( {self.text()} )')
-        ev = QKeyEvent(QEvent.Type.KeyPress, Qt.Key.Key_Enter, Qt.KeyboardModifier.NoModifier)
-        self.keyPressEvent(ev)
+    def invert_search(self):
+        q = self.text().strip()
+        if q:
+            if q.startswith('NOT ( ') and q.endswith(' )'):
+                q = q[6:-2]
+            else:
+                q = f'NOT ( {q} )'
+            self.setText(q)
+            ev = QKeyEvent(QEvent.Type.KeyPress, Qt.Key.Key_Enter, Qt.KeyboardModifier.NoModifier)
+            self.keyPressEvent(ev)
 
     def paste_and_search(self):
         self.paste()