diff --git a/phiml/math/_ops.py b/phiml/math/_ops.py
index 6d5df7d..b39b12a 100644
--- a/phiml/math/_ops.py
+++ b/phiml/math/_ops.py
@@ -1678,11 +1678,7 @@ def lookup_where(native_index_fn: Callable, value: Union[Tensor, PhiTreeNode, No
     idx_native = native_index_fn(key_nat)
     multi_idx_native = choose_backend(idx_native).unravel_index(idx_native[:, 0], dims.sizes)
     idx = reshaped_tensor(multi_idx_native, [key_batch, channel(_index=dims)])
-    def lookup(t: Tensor):
-        if t is None:
-            return None
-        return gather(t, idx, pref_index_dim='_index')
-    return tree_map(lookup, value)
+    return tree_map(lambda t: gather(t, idx, pref_index_dim='_index'), value)
 
 
 def at_max(value, key: Tensor, dim: DimFilter = non_batch):
@@ -2485,6 +2481,8 @@ def gather(values, indices: Tensor, dims: Union[DimFilter, None] = None, pref_in
     Returns:
         `Tensor` with combined batch dimensions, channel dimensions of `values` and spatial/instance dimensions of `indices`.
     """
+    if values is None:
+        return None
     if not isinstance(values, Tensor):
         return tree_map(lambda v: gather(v, indices, dims), values)
     index_dim = channel(indices)