diff --git a/dmi.c b/dmi.c
index a7a9a61..832d7ab 100644
--- a/dmi.c
+++ b/dmi.c
@@ -142,7 +142,7 @@
 
 	/* fix junk bytes in the string */
 	for (i = 0; i < len && buf[i] != '\0'; i++) {
-		if (isprint(buf[i]))
+		if (isprint((unsigned char)buf[i]))
 			newbuf[i] = buf[i];
 		else
 			newbuf[i] = ' ';