diff --git a/os/hal/include/hal_uart.h b/os/hal/include/hal_uart.h
index 9e5d2a373..a07464993 100644
--- a/os/hal/include/hal_uart.h
+++ b/os/hal/include/hal_uart.h
@@ -43,6 +43,13 @@
 #define UART_BREAK_DETECTED     64  /**< @brief Break detected.             */
 /** @} */
 
+/**
+ * @name    UART error conditions
+ * @{
+ */
+#define UART_ERR_NOT_ACTIVE     (size_t)-1
+/** @} */
+
 /*===========================================================================*/
 /* Driver pre-compile time settings.                                         */
 /*===========================================================================*/
diff --git a/os/hal/src/hal_uart.c b/os/hal/src/hal_uart.c
index dd9f4a6d2..2d6a4c408 100644
--- a/os/hal/src/hal_uart.c
+++ b/os/hal/src/hal_uart.c
@@ -188,7 +188,7 @@ void uartStartSendI(UARTDriver *uartp, size_t n, const void *txbuf) {
  *
  * @return              The number of data frames not transmitted by the
  *                      stopped transmit operation.
- * @retval 0            There was no transmit operation in progress.
+ * @retval UART_ERR_NOT_ACTIVE if there was no transmit operation in progress.
  *
  * @api
  */
@@ -205,7 +205,7 @@ size_t uartStopSend(UARTDriver *uartp) {
     uartp->txstate = UART_TX_IDLE;
   }
   else {
-    n = 0;
+    n = UART_ERR_NOT_ACTIVE;
   }
   osalSysUnlock();
 
@@ -221,7 +221,7 @@ size_t uartStopSend(UARTDriver *uartp) {
  *
  * @return              The number of data frames not transmitted by the
  *                      stopped transmit operation.
- * @retval 0            There was no transmit operation in progress.
+ * @retval UART_ERR_NOT_ACTIVE if there was no transmit operation in progress.
  *
  * @iclass
  */
@@ -236,7 +236,7 @@ size_t uartStopSendI(UARTDriver *uartp) {
     uartp->txstate = UART_TX_IDLE;
     return n;
   }
-  return 0;
+  return UART_ERR_NOT_ACTIVE;
 }
 
 /**
@@ -294,7 +294,7 @@ void uartStartReceiveI(UARTDriver *uartp, size_t n, void *rxbuf) {
  *
  * @return              The number of data frames not received by the
  *                      stopped receive operation.
- * @retval 0            There was no receive operation in progress.
+ * @retval UART_ERR_NOT_ACTIVE if there was no receive operation in progress.
  *
  * @api
  */
@@ -311,7 +311,7 @@ size_t uartStopReceive(UARTDriver *uartp) {
     uartp->rxstate = UART_RX_IDLE;
   }
   else {
-    n = 0;
+    n = UART_ERR_NOT_ACTIVE;
   }
   osalSysUnlock();
 
@@ -327,7 +327,7 @@ size_t uartStopReceive(UARTDriver *uartp) {
  *
  * @return              The number of data frames not received by the
  *                      stopped receive operation.
- * @retval 0            There was no receive operation in progress.
+ * @retval UART_ERR_NOT_ACTIVE if there was no receive operation in progress.
  *
  * @iclass
  */
@@ -342,7 +342,7 @@ size_t uartStopReceiveI(UARTDriver *uartp) {
     uartp->rxstate = UART_RX_IDLE;
     return n;
   }
-  return 0;
+  return UART_ERR_NOT_ACTIVE;
 }
 
 #if (UART_USE_WAIT == TRUE) || defined(__DOXYGEN__)
diff --git a/readme.txt b/readme.txt
index 746c2d37e..f8708db89 100644
--- a/readme.txt
+++ b/readme.txt
@@ -126,6 +126,8 @@
 - EX:  Updated LIS302DL to 1.1.0 (backported to 18.2.1).
 - EX:  Updated LPS25H to 1.1.0 (backported to 18.2.1).
 - EX:  Updated LSM303DLHC to 1.1.0 (backported to 18.2.1).
+- HAL: Fixed missing definition in UART driver (bug #948)(backported
+       to 18.2.2).
 - OTH: Fixed wrong macro check in GCC Cortex-M startup files (bug #947)
        (backported to 18.2.2 and 17.6.5).
 - HAL: Fixed binary instead of logic operator in STM32F4 HAL (bug #946)