--- a/sys/bus/isa/pnp.c
+++ b/sys/bus/isa/pnp.c
@@ -403,7 +403,7 @@
 				/*
 				 * Trim trailing spaces.
 				 */
-				while (buf[large_len-1] == ' ')
+				while (large_len > 0 && buf[large_len-1] == ' ')
 					large_len--;
 				buf[large_len] = '\0';
 				desc = buf;