diff --git a/phiml/math/_sparse.py b/phiml/math/_sparse.py
index 1902c871..21bcc71c 100644
--- a/phiml/math/_sparse.py
+++ b/phiml/math/_sparse.py
@@ -762,9 +762,9 @@ def _native_csr_components(self, invalid='clamp', get_values=True):
         assert invalid in ['clamp', 'discard', 'keep']
         ind_batch = batch(self._indices) & batch(self._pointers)
         channels = non_instance(self._values).without(ind_batch)
-        native_indices = self._indices.native([ind_batch, instance])
-        native_pointers = self._pointers.native([ind_batch, instance])
-        native_values = self._values.native([ind_batch, instance, channels]) if get_values else None
+        native_indices = self._indices._reshaped_native([ind_batch, instance(self._indices).without_sizes()])  # allow variable instance size (PyTorch tracing)
+        native_pointers = self._pointers._reshaped_native([ind_batch, instance(self._pointers)])
+        native_values = self._values._reshaped_native([ind_batch, instance(self._values).without_sizes(), channels]) if get_values else None
         native_shape = self._compressed_dims.volume, self._uncompressed_dims.volume
         if self._uncompressed_offset is not None:
             native_indices -= self._uncompressed_offset
diff --git a/phiml/math/_tensors.py b/phiml/math/_tensors.py
index f0613afa..1301f0a7 100644
--- a/phiml/math/_tensors.py
+++ b/phiml/math/_tensors.py
@@ -1228,7 +1228,7 @@ def __init__(self, native_tensor, names: Sequence[str], expanded_shape: Shape, b
                     for s_dim in dim.size.shape.names:
                         assert s_dim in expanded_shape.names, f"Dimension {dim} varies along {s_dim} but {s_dim} is not part of the Shape {self}"
             assert choose_backend(native_tensor) == backend
-            assert expanded_shape.is_uniform
+            assert expanded_shape.is_uniform, expanded_shape
             shape_sizes = [expanded_shape.get_size(n) for n in names]
             assert backend.staticshape(native_tensor) == tuple(shape_sizes), f"Shape {expanded_shape} at {names} does not match native tensor with shape {backend.staticshape(native_tensor)}"
 
@@ -1264,7 +1264,7 @@ def _reshaped_native(self, groups: Sequence[Shape]):
         native = self._backend.transpose(self._native, perm)  # this will cast automatically
         native = native[tuple(slices)]
         native = self._backend.tile(native, tile)
-        native = self._backend.reshape(native, [g.volume for g in groups])
+        native = self._backend.reshape(native, [g.volume if g.well_defined else -1 for g in groups])
         return native
 
     def _transposed_native(self, order: Sequence[str], force_expand: bool):