diff --git a/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c b/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c
index 4014ab5bb..b71fa377a 100644
--- a/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c
+++ b/os/hal/ports/STM32/LLD/USARTv2/hal_serial_lld.c
@@ -158,7 +158,7 @@ static uint8_t sd_out_buf3[STM32_SERIAL_USART3_OUT_BUF_SIZE];
 static uint8_t sd_in_buf4[STM32_SERIAL_UART4_IN_BUF_SIZE];
 
 /** @brief Output buffer for SD4.*/
-static uint8_t sd_out_buf4[STM32_SERIAL_UART4_IN_BUF_SIZE];
+static uint8_t sd_out_buf4[STM32_SERIAL_UART4_OUT_BUF_SIZE];
 #endif
 
 #if STM32_SERIAL_USE_UART5 || defined(__DOXYGEN__)
@@ -166,7 +166,7 @@ static uint8_t sd_out_buf4[STM32_SERIAL_UART4_IN_BUF_SIZE];
 static uint8_t sd_in_buf5[STM32_SERIAL_UART5_IN_BUF_SIZE];
 
 /** @brief Output buffer for SD5.*/
-static uint8_t sd_out_buf5[STM32_SERIAL_UART5_IN_BUF_SIZE];
+static uint8_t sd_out_buf5[STM32_SERIAL_UART5_OUT_BUF_SIZE];
 #endif
 
 #if STM32_SERIAL_USE_USART6 || defined(__DOXYGEN__)
diff --git a/readme.txt b/readme.txt
index 7d5316fcf..e302972f4 100644
--- a/readme.txt
+++ b/readme.txt
@@ -89,6 +89,8 @@
 *****************************************************************************
 
 *** 17.6.1 ***
+- HAL: Fixed STM32 USARTv2 serial incorrect buffer size declarations
+       (bug #871).
 - HAL: Fixed board file configuration for STM32F3 Discovery REVC (bug #869).
 - HAL: Fixed wrong PPRE2 and LSI related macros in STM32L0 hal lld (bug #868).
 - HAL: Fixed wrong bit mask in STM32L0 hal lld (bug #866).