diff --git a/os/hal/ports/STM32/LLD/USARTv2/hal_uart_lld.c b/os/hal/ports/STM32/LLD/USARTv2/hal_uart_lld.c
index 5bf3b6089..54378ff06 100644
--- a/os/hal/ports/STM32/LLD/USARTv2/hal_uart_lld.c
+++ b/os/hal/ports/STM32/LLD/USARTv2/hal_uart_lld.c
@@ -127,6 +127,11 @@
 #define UART8 USART8
 #endif
 
+/* Workaround for more differences in headers.*/
+#if !defined(USART_CR1_M0)
+#define USART_CR1_M0 USART_CR1_M
+#endif
+
 /*===========================================================================*/
 /* Driver exported variables.                                                */
 /*===========================================================================*/
@@ -845,7 +850,7 @@ void uart_lld_start(UARTDriver *uartp) {
 
     /* Static DMA setup, the transfer size depends on the USART settings,
        it is 16 bits if M=1 and PCE=0 else it is 8 bits.*/
-    if ((uartp->config->cr1 & (USART_CR1_M | USART_CR1_PCE)) == USART_CR1_M)
+    if ((uartp->config->cr1 & (USART_CR1_M | USART_CR1_PCE)) == USART_CR1_M0)
       uartp->dmamode |= STM32_DMA_CR_PSIZE_HWORD | STM32_DMA_CR_MSIZE_HWORD;
     dmaStreamSetPeripheral(uartp->dmarx, &uartp->usart->RDR);
     dmaStreamSetPeripheral(uartp->dmatx, &uartp->usart->TDR);
diff --git a/readme.txt b/readme.txt
index ace1e6835..06a8e9227 100644
--- a/readme.txt
+++ b/readme.txt
@@ -88,6 +88,8 @@
 *****************************************************************************
 
 *** Next ***
+- HAL: Fixed wrong frame size code in STM32 USARTv2 UART driver (bug #851)
+       (backported to 17.6.1 and 16.1.9).
 - NIL: Fixed documentation-related issues (bug #850)(backported to 17.6.1).
 - RT:  Fixed documentation-related issues (bug #850)(backported to 17.6.1).
 - RT:  Fixed leftover chcustomer.h file (bug #849)(backported to 17.6.1).