workaround missing mask problem

spectral_cube/cube_utils.py

@@ -1169,6 +1169,13 @@ def update(self, n=1):
         dx = outwcs.spectral.proj_plane_pixel_scales()[0]
         log_(f"Channel mode: dx={dx}.  Looping over {len(channels)} channels and {len(cubes)} cubes")
 
+        # workaround: some cubes exist with no mask, which breaks downstream when 'None' is passed into
+        # subcube_slices_from_mask.  In theory, this operation is lazy and therefore cheap
+        for cube in cubes:
+            if cube.mask is None:
+                # cube == cube is a lazy shorthand for 'isfinite'
+                cube = cube.with_mask(cube == cube)
+
         mincube_slices = [cube[cube.shape[0]//2:cube.shape[0]//2+1]
                           .subcube_slices_from_mask(cube[cube.shape[0]//2:cube.shape[0]//2+1].mask,
                                                     spatial_only=True)