diff --git a/libraries/AP_IOMCU/AP_IOMCU.cpp b/libraries/AP_IOMCU/AP_IOMCU.cpp
index a01cd33316..cf5b965364 100644
--- a/libraries/AP_IOMCU/AP_IOMCU.cpp
+++ b/libraries/AP_IOMCU/AP_IOMCU.cpp
@@ -781,7 +781,7 @@ bool AP_IOMCU::check_crc(void)
     if (io_crc == crc) {
         hal.console->printf("IOMCU: CRC ok\n");
         crc_is_ok = true;
-        free(fw);
+        AP_ROMFS::free(fw);
         fw = nullptr;
         return true;
     } else {
@@ -792,12 +792,12 @@ bool AP_IOMCU::check_crc(void)
     write_registers(PAGE_SETUP, PAGE_REG_SETUP_REBOOT_BL, 1, &magic);
 
     if (!upload_fw()) {
-        free(fw);
+        AP_ROMFS::free(fw);
         fw = nullptr;
         AP_BoardConfig::sensor_config_error("Failed to update IO firmware");
     }
 
-    free(fw);
+    AP_ROMFS::free(fw);
     fw = nullptr;
     return false;
 }
diff --git a/libraries/AP_IOMCU/AP_IOMCU.h b/libraries/AP_IOMCU/AP_IOMCU.h
index c02721061e..06b745decb 100644
--- a/libraries/AP_IOMCU/AP_IOMCU.h
+++ b/libraries/AP_IOMCU/AP_IOMCU.h
@@ -215,7 +215,7 @@ private:
 
     // firmware upload
     const char *fw_name = "io_firmware.bin";
-    uint8_t *fw;
+    const uint8_t *fw;
     uint32_t fw_size;
 
     size_t write_wait(const uint8_t *pkt, uint8_t len);