diff --git a/climax/main/preprocess.py b/climax/main/preprocess.py
index f6b537db302185db91368e48e9d7f38b2d184474..bf396b61a4e097fe1d762e35870a01e22c483946 100644
--- a/climax/main/preprocess.py
+++ b/climax/main/preprocess.py
@@ -164,7 +164,7 @@ if __name__ == '__main__':
                 # the current simulation
                 y_min, _ = _parse_cordex_time_span(group[0])  # first year
                 _, y_max = _parse_cordex_time_span(group[-1])  # last year
-                filename = '_'.join([sim, '-'.join([y_min, y_max])])
+                filename = '_'.join([sim, '-'.join([y_min, y_max])]) + '.nc'
                 filename = group[0].parent.joinpath(filename)
 
                 # log simulation name, time span and files