diff --git a/lark/visitors.py b/lark/visitors.py
index 801e630c..44f61b01 100644
--- a/lark/visitors.py
+++ b/lark/visitors.py
@@ -156,11 +156,11 @@ def _transform_tree(self, tree):
         children = list(self._transform_children(tree.children))
         return self._call_userfunc(tree, children)
 
-    def transform(self, tree: Union[Tree[_Leaf_T], _Leaf_T]) -> Optional[_Return_T]:
+    def transform(self, tree: Tree[_Leaf_T]) -> _Return_T:
         "Transform the given tree, and return the final result"
         res = list(self._transform_children([tree]))
         if not res:
-            return
+            raise RuntimeError("Cannot discard the root node")
         assert len(res) == 1
         return res[0]