diff --git a/sys/dev/agp/agp_sis.c b/sys/dev/agp/agp_sis.c
@@ -212,7 +212,7 @@
 	    || aperture > 256*1024*1024)
 		return EINVAL;
 
-	gws = ffs(aperture / 4*1024*1024) - 1;
+	gws = ffs(aperture / (4*1024*1024)) - 1;
 	
 	pci_write_config(dev, AGP_SIS_WINCTRL,
 			 ((pci_read_config(dev, AGP_SIS_WINCTRL, 1) & ~0x70)