diff --git a/src/xfel_calibrate/calibrate.py b/src/xfel_calibrate/calibrate.py
index 103258b3f8d1836b95babc1470e1d8d23cc16a9e..ebc1a54cd2dec6c859d949f530cf739447607a64 100755
--- a/src/xfel_calibrate/calibrate.py
+++ b/src/xfel_calibrate/calibrate.py
@@ -585,7 +585,7 @@ def run(argv=None):
     """ Run a calibration task with parser arguments """
     # Ensure files are opened as UTF-8 by default, regardless of environment.
     if "readthedocs.org" not in sys.executable:
-        locale.setlocale(locale.LC_CTYPE, ('en_US', 'UTF-8'))
+        locale.setlocale(locale.LC_CTYPE, ('C', 'UTF-8'))
 
     if argv is None:
         argv = sys.argv