diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index b6d183a..2e01a9d 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -74,9 +74,10 @@ static void sg_proc_cleanup(void);
 #define SG_MAX_DEVS 32768
 
 /* SG_MAX_CDB_SIZE should be 260 (spc4r37 section 3.1.30) however the type
- * of sg_io_hdr::cmd_len can only represent 255
+ * of sg_io_hdr::cmd_len can only represent 255. All SCSI commands greater
+ * than 16 bytes are "variable length" whose length is a multiple of 4
  */
-#define SG_MAX_CDB_SIZE 255
+#define SG_MAX_CDB_SIZE 252
 
 /*
  * Suppose you want to calculate the formula muldiv(x,m,d)=int(x * m / d)