--- ispell-3.1.20.orig/ispell.c
+++ ispell-3.1.20/ispell.c
@@ -256,6 +256,7 @@
     static char	outbuf[BUFSIZ];
     int		argno;
     int		arglen;
+    int		bflag = 0;
     int		dictdeft = 0;
 
     Cmd = *argv;
@@ -552,12 +553,13 @@
 		nodictflag++;
 		break;
 	    case 'b':
-		if (arglen > 2)
+		if (arglen > 2 || xflag == 1)
 		    usage ();
 		xflag = 0;		/* Keep a backup file */
+		bflag = 1;
 		break;
 	    case 'x':
-		if (arglen > 2)
+		if (arglen > 2 || bflag == 1)
 		    usage ();
 		xflag = 1;		/* Don't keep a backup file */
 		break;