diff --git a/include/ac/ctype.h b/include/ac/ctype.h
index b58d96fc6f..b5638fe97d 100644
--- a/include/ac/ctype.h
+++ b/include/ac/ctype.h
@@ -16,7 +16,7 @@
 
 #ifdef C_UPPER_LOWER
 # define TOUPPER(c)	(islower(c) ? toupper(c) : (c))
-# define TOLOWER(c)	(islower(c) ? toupper(c) : (c))
+# define TOLOWER(c)	(isupper(c) ? tolower(c) : (c))
 #else
 # define TOUPPER(c)	toupper(c)
 # define TOLOWER(c)	tolower(c)