diff --git a/commands/nand.c b/commands/nand.c
index ad1c8c9..c330ad1 100644
--- a/commands/nand.c
+++ b/commands/nand.c
@@ -90,8 +90,13 @@
 		}
 
 		ret = ioctl(fd, MEMSETBADBLOCK, &badblock);
-		if (ret)
-			perror("ioctl");
+		if (ret) {
+			if (ret == -EINVAL)
+				printf("Maybe offset %lld is out of range.\n",
+					badblock);
+			else
+				perror("ioctl");
+		}
 
 		close(fd);
 		return ret;
diff --git a/drivers/mtd/core.c b/drivers/mtd/core.c
index 9234525..e35571d 100644
--- a/drivers/mtd/core.c
+++ b/drivers/mtd/core.c
@@ -309,6 +309,9 @@
 {
 	int ret;
 
+	if (ofs < 0 || ofs >= mtd->size)
+		return -EINVAL;
+
 	if (mtd->block_markbad)
 		ret = mtd->block_markbad(mtd, ofs);
 	else