diff --git a/include/ctypef.c b/include/ctypef.c
index 7cd10fc..9fb3dfb 100644
--- a/include/ctypef.c
+++ b/include/ctypef.c
@@ -49,7 +49,7 @@ int
 _ispunct(int c){ return c != ' ' && !isalnum(c) };
 
 int
-_tolower(int c){ return c -= isupper(c) * ('A' - 'a'); }
+_tolower(int c){ return c - isupper(c) * ('A' - 'a'); }
 
 int
-_toupper(int c){ return c += islower(c) * ('A' - 'a'); }
+_toupper(int c){ return c + islower(c) * ('A' - 'a'); }