diff --git a/aqt/browser.py b/aqt/browser.py
index 75d5eaf62..91a596252 100644
--- a/aqt/browser.py
+++ b/aqt/browser.py
@@ -394,7 +394,7 @@ class SidebarItem:
         self.expanded = expanded
         self.children: List["SidebarItem"] = []
         self.parentItem: Optional[SidebarItem] = None
-        self.tooltip = None
+        self.tooltip: Optional[str] = None
 
     def addChild(self, cb: "SidebarItem") -> None:
         self.children.append(cb)