diff --git a/lib/libusb_stm32/src/usbd_stm32l433_devfs.c b/lib/libusb_stm32/src/usbd_stm32l433_devfs.c
index 5b3f95381..c1463c377 100644
--- a/lib/libusb_stm32/src/usbd_stm32l433_devfs.c
+++ b/lib/libusb_stm32/src/usbd_stm32l433_devfs.c
@@ -336,7 +336,7 @@ static int32_t ep_read(uint8_t ep, void *buf, uint16_t blen) {
 }
 
 static void pma_write(const uint8_t *buf, uint16_t blen, pma_rec *tx) {
-    uint16_t *pma = (void*)(USB_PMAADDR + tx->addr);
+    volatile uint16_t *pma = (void*)(USB_PMAADDR + tx->addr);
     tx->cnt = blen;
     while (blen > 1) {
         *pma++ = buf[1] << 8 | buf[0];