diff --git a/rtos/source/Mutex.cpp b/rtos/source/Mutex.cpp
index 7752f6670cd..57e69f7c9eb 100644
--- a/rtos/source/Mutex.cpp
+++ b/rtos/source/Mutex.cpp
@@ -97,6 +97,7 @@ bool Mutex::trylock_for(uint32_t millisec)
 {
     osStatus status = osMutexAcquire(_id, millisec);
     if (status == osOK) {
+        _count++;
         return true;
     }