diff --git a/tinygrad/mixin/movement.py b/tinygrad/mixin/movement.py
index dc03177ec9d4b..4e750f7fb22ae 100644
--- a/tinygrad/mixin/movement.py
+++ b/tinygrad/mixin/movement.py
@@ -329,6 +329,7 @@ def flatten(self, start_dim=0, end_dim=-1) -> Self:
     ```
     """
     start_dim, end_dim = self._resolve_dim(start_dim), self._resolve_dim(end_dim)
+    if start_dim > end_dim: raise RuntimeError(f"flatten: {start_dim=} > {end_dim=}")
     return self.reshape(self.shape[:start_dim] + (prod(self.shape[start_dim : end_dim + 1]),) + self.shape[end_dim + 1 :])
 
   def unflatten(self, dim: int, sizes: tuple[int, ...]) -> Self: