diff --git a/libraries/AP_IOMCU/iofirmware/ioprotocol.h b/libraries/AP_IOMCU/iofirmware/ioprotocol.h
index 2835c00fe3..7dbf319e66 100644
--- a/libraries/AP_IOMCU/iofirmware/ioprotocol.h
+++ b/libraries/AP_IOMCU/iofirmware/ioprotocol.h
@@ -162,7 +162,7 @@ struct page_mixing {
     uint8_t pad; // pad to even size
 };
 
-struct page_GPIO {
+struct __attribute__((packed, aligned(2))) page_GPIO {
     uint8_t channel_mask;
     uint8_t output_mask;
 };