diff --git a/lib/mu-str.c b/lib/mu-str.c
index a22c8b6f..11801be9 100644
--- a/lib/mu-str.c
+++ b/lib/mu-str.c
@@ -839,7 +839,7 @@ mu_str_asciify_in_place (char *buf)
 	g_return_val_if_fail (buf, NULL);
 
 	for (c = buf; c && *c; ++c) {
-		if (!isascii(*c) || *c < ' ')
+		if (!isprint(*c) && !isspace (*c))
 			*c = '.';
 	}