diff --git a/common/cmd_mmc.c b/common/cmd_mmc.c
index 8c53a10315ecec188d75d87c15b67a1971e59aa7..9f3d6c575bfe2ef8c12b54d4fb1f0e42e2eacfa0 100644
--- a/common/cmd_mmc.c
+++ b/common/cmd_mmc.c
@@ -164,8 +164,12 @@ static int do_mmcops(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 	}
 
 	if (strcmp(argv[1], "rescan") == 0) {
-		struct mmc *mmc = find_mmc_device(curr_device);
+		struct mmc *mmc;
 
+		if (argc != 2)
+			return CMD_RET_USAGE;
+
+		mmc = find_mmc_device(curr_device);
 		if (!mmc) {
 			printf("no mmc device at slot %x\n", curr_device);
 			return 1;
@@ -179,8 +183,12 @@ static int do_mmcops(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 			return 0;
 	} else if (strncmp(argv[1], "part", 4) == 0) {
 		block_dev_desc_t *mmc_dev;
-		struct mmc *mmc = find_mmc_device(curr_device);
+		struct mmc *mmc;
+
+		if (argc != 2)
+			return CMD_RET_USAGE;
 
+		mmc = find_mmc_device(curr_device);
 		if (!mmc) {
 			printf("no mmc device at slot %x\n", curr_device);
 			return 1;
@@ -196,6 +204,8 @@ static int do_mmcops(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 		puts("get mmc type error!\n");
 		return 1;
 	} else if (strcmp(argv[1], "list") == 0) {
+		if (argc != 2)
+			return CMD_RET_USAGE;
 		print_mmc_devices('\n');
 		return 0;
 	} else if (strcmp(argv[1], "dev") == 0) {