diff --git a/ssh.c b/ssh.c
index 2681ccb..0c95074 100644
--- a/ssh.c
+++ b/ssh.c
@@ -629,7 +629,7 @@
 	if (options.host_key_alias != NULL) {
 		for (p = options.host_key_alias; *p; p++)
 			if (isupper(*p))
-				*p = tolower(*p);
+				*p = (char)tolower(*p);
 	}
 
 	/* Get default port if port has not been set. */