diff --git a/Makefile b/Makefile
index 429e6cf09474ba60ad0de80bc9ee99b614875f3f..a797681b1cdf1081bf54abbc5c05d89b32d7181e 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,7 @@ tmp generated:
 TJ3: generated/timestamp
 
 generated/timestamp: planning.tjp $(TJI) | generated
-	LC_CTYPE=C.UTF-8 tj3 -o generated planning.tjp
+	LC_CTYPE=en_US.UTF-8 tj3 -o generated planning.tjp
 	touch $@
 
 pre-commit: TJ3