Update raw_metadata.py

nd2reader/raw_metadata.py

@@ -163,7 +163,11 @@ class RawMetadata(object):
         """
         z_levels = self._parse_dimension(r""".*?Z\((\d+)\).*?""")
         if 0 == len(z_levels):
-            z_levels = range(len(parse_if_not_none(self.z_data, self._parse_z_coordinates)))
+            z_levels = parse_if_not_none(self.z_data, self._parse_z_coordinates)
+            if isinstance(z_levels, type(None)):
+                z_levels = []
+            else:
+                z_levels = range(len(z_levels))
             if 0 != z_levels:
                 warnings.warn("Z-levels details missing in metadata. Using Z-coordinates instead.")
         return z_levels