diff --git a/os/hal/src/mac.c b/os/hal/src/mac.c
index 2cafbcdbc..365c3c420 100644
--- a/os/hal/src/mac.c
+++ b/os/hal/src/mac.c
@@ -130,8 +130,10 @@ msg_t macWaitTransmitDescriptor(MACDriver *macp,
          (time > 0)) {
     chSysLock();
     systime_t now = chTimeNow();
-    if ((msg = chSemWaitTimeoutS(&macp->md_tdsem, time)) == RDY_TIMEOUT)
+    if ((msg = chSemWaitTimeoutS(&macp->md_tdsem, time)) == RDY_TIMEOUT) {
+      chSysUnlock();
       break;
+    }
     if (time != TIME_INFINITE)
       time -= (chTimeNow() - now);
     chSysUnlock();
@@ -180,8 +182,10 @@ msg_t macWaitReceiveDescriptor(MACDriver *macp,
          (time > 0)) {
     chSysLock();
     systime_t now = chTimeNow();
-    if ((msg = chSemWaitTimeoutS(&macp->md_rdsem, time)) == RDY_TIMEOUT)
+    if ((msg = chSemWaitTimeoutS(&macp->md_rdsem, time)) == RDY_TIMEOUT) {
+      chSysUnlock();
       break;
+    }
     if (time != TIME_INFINITE)
       time -= (chTimeNow() - now);
     chSysUnlock();
diff --git a/readme.txt b/readme.txt
index 5499817d1..2485d3c8d 100644
--- a/readme.txt
+++ b/readme.txt
@@ -69,6 +69,7 @@
 *****************************************************************************
 
 *** 2.2.1 ***
+- FIX: Error in MAC driver (bug 3179783).
 - FIX: Fixed wrong serial driver macros (bug 3173336).
 
 *** 2.2.0 ***