diff --git a/volia/masseffect.py b/volia/masseffect.py
index a54e394..e83790b 100644
--- a/volia/masseffect.py
+++ b/volia/masseffect.py
@@ -99,7 +99,7 @@ def utt2sub(file: str, subfile: str, outfile: str):
     data_sub = core.data.read_id_values(subfile)
     keys_sub = [key for key in data_sub]
 
-    with open(outfile) as of:
+    with open(outfile, "w") as of:
         for key in keys:
             subkeys = [subkey for subkey in keys_sub if subkey.startswith(key)]
             subkeys_str = " ".join(subkeys)
@@ -113,7 +113,7 @@ def sub2utt(file: str, subfile: str, outfile: str):
     data_sub = core.data.read_id_values(subfile)
     keys_sub = [key for key in data_sub]
 
-    with open(outfile) as of:
+    with open(outfile, "w") as of:
         for key in keys:
             subkeys = [subkey for subkey in keys_sub if subkey.startswith(key)]
             for subkey in subkeys: