diff --git a/trusty/storage/proxy/rpmb.c b/trusty/storage/proxy/rpmb.c
index f059935..b1b8232 100644
--- a/trusty/storage/proxy/rpmb.c
+++ b/trusty/storage/proxy/rpmb.c
@@ -322,9 +322,9 @@
 }
 
 static int send_mmc_rpmb_req(int mmc_fd, const struct storage_rpmb_send_req* req) {
-    struct {
+    union {
         struct mmc_ioc_multi_cmd multi;
-        struct mmc_ioc_cmd cmd_buf[3];
+        uint8_t raw[sizeof(struct mmc_ioc_multi_cmd) + sizeof(struct mmc_ioc_cmd) * 3];
     } mmc = {};
     struct mmc_ioc_cmd* cmd = mmc.multi.cmds;
     int rc;