diff --git a/src/dosinst.c b/src/dosinst.c
index 4eae5aa..35625a7 100644
--- a/src/dosinst.c
+++ b/src/dosinst.c
@@ -2760,7 +2760,7 @@
 	    rewind(stdin);
 	    if (scanf("%99s", buf) == 1)
 	    {
-		if (isdigit(buf[0]))
+		if (isdigit((unsigned char)buf[0]))
 		{
 		    // Change a choice.
 		    i = atoi(buf);