diff --git a/climax/main/preprocess.py b/climax/main/preprocess.py
index bf396b61a4e097fe1d762e35870a01e22c483946..23bce1afc9252b381435b400590ec1ef11b4056e 100644
--- a/climax/main/preprocess.py
+++ b/climax/main/preprocess.py
@@ -174,7 +174,7 @@ if __name__ == '__main__':
                     ['{}'.format(file) for file in group]))
 
                 # read multiple netcdf files using xarray and dask
-                ds = xr.open_mfdataset(group)
+                ds = xr.open_mfdataset(group).load()
 
                 # save aggregated netcdf file
                 LOGGER.info('Saving aggregated NetCDF: {}'.format(filename))