diff --git a/elf/io/image_stack_wrapper.py b/elf/io/image_stack_wrapper.py
index 84f7c55..385d65b 100644
--- a/elf/io/image_stack_wrapper.py
+++ b/elf/io/image_stack_wrapper.py
@@ -165,7 +165,7 @@ def _load_roi_from_slices(self, roi):
         def _load_and_write_image(z):
             z_abs = z + z0
             im = self._read_image(z_abs)
-            assert im.shape == self.im_shape
+            assert im.shape == self.im_shape, f"{im.shape}, {self.im_shape}"
             data[z] = im[im_roi]
 
         # load the slices and write them into the output data
@@ -209,7 +209,8 @@ def is_tif_slices(files):
         if ext.lower() not in TifStackDataset.tif_exts:
             return False
         try:
-            tifffile.memmap(f0, mode="r")
+            for ff in files:
+                tifffile.memmap(ff, mode="r")
         except ValueError:
             return False
         return True
diff --git a/elf/io/mrc_wrapper.py b/elf/io/mrc_wrapper.py
index 0a060e1..215166f 100755
--- a/elf/io/mrc_wrapper.py
+++ b/elf/io/mrc_wrapper.py
@@ -33,7 +33,7 @@ def shape(self):
         return self._data.shape
 
     def __getitem__(self, key):
-        return self._data[key]
+        return self._data[key].copy()
 
     @property
     def size(self):