diff --git a/spectrum.py b/spectrum.py
index 612d2d1..3b57daa 100644
--- a/spectrum.py
+++ b/spectrum.py
@@ -116,7 +116,7 @@ cache_file = pathlib.Path( cache )
 if cache_file.is_file() and not no_cache:
     if verbose:
         print( 'using cache' )
-    with shelve.optn( str( cache_file ) ) as cache:
+    with shelve.open( str( cache_file ) ) as cache:
         for key in [ 'data' , 'border' , 'calibrations' ]:
             if key not in cache:
                 raise Exception( 'spectrum.py: missing data in cache file' )