diff --git a/octant/roms.py b/octant/roms.py
index 947c7c0..e729ef3 100644
--- a/octant/roms.py
+++ b/octant/roms.py
@@ -281,7 +281,9 @@ def __init__(self, nc, grid, ncg=None):
         self.nc = nc
         if ncg is None:
             self.ncg = self.nc
-        
+        else:
+            self.ncg = ncg
+
         # Get the vertical dimension of the grid.
         if 'N' in self.nc.dimensions.keys():
             self.N = len(self.nc.dimensions['N'])