diff --git a/source/ff.c b/source/ff.c
index cbdf685..22a7cb9 100644
--- a/source/ff.c
+++ b/source/ff.c
@@ -5957,7 +5957,7 @@ FRESULT f_mkfs (
 		sz_fat = (DWORD)((sz_vol / sz_au + 2) * 4 + ss - 1) / ss;	/* Number of FAT sectors */
 		b_data = (b_fat + sz_fat + sz_blk - 1) & ~((LBA_t)sz_blk - 1);	/* Align data area to the erase block boundary */
 		if (b_data - b_vol >= sz_vol / 2) LEAVE_MKFS(FR_MKFS_ABORTED);	/* Too small volume? */
-		n_clst = (DWORD)(sz_vol - (b_data - b_vol)) / sz_au;	/* Number of clusters */
+		n_clst = (DWORD)((sz_vol - (b_data - b_vol)) / sz_au);	/* Number of clusters */
 		if (n_clst <16) LEAVE_MKFS(FR_MKFS_ABORTED);			/* Too few clusters? */
 		if (n_clst > MAX_EXFAT) LEAVE_MKFS(FR_MKFS_ABORTED);	/* Too many clusters? */