--- a/sys/dev/agp/agp.c
+++ b/sys/dev/agp/agp.c
@@ -857,7 +857,7 @@
 	device_t dev = kdev->si_drv1;
 	struct agp_softc *sc = device_get_softc(dev);
 
-	if (ap->a_offset > AGP_GET_APERTURE(dev))
+	if (ap->a_offset + PAGE_SIZE > AGP_GET_APERTURE(dev) + 1)
 		return EINVAL;
 	ap->a_result = atop(rman_get_start(sc->as_aperture) + ap->a_offset);
 	return 0;