diff --git a/rtos/Mutex.cpp b/rtos/Mutex.cpp
index 56770399d6..f384bce115 100644
--- a/rtos/Mutex.cpp
+++ b/rtos/Mutex.cpp
@@ -108,10 +108,13 @@ bool Mutex::trylock_until(uint64_t millisec)
     }
 }
 
-osStatus Mutex::unlock()
+void Mutex::unlock()
 {
     _count--;
-    return osMutexRelease(_id);
+
+    osStatus status = osMutexRelease(_id);
+
+    MBED_ASSERT(status == osOK);
 }
 
 osThreadId Mutex::get_owner()
diff --git a/rtos/Mutex.h b/rtos/Mutex.h
index dc20ad2bdd..4c738b7e7b 100644
--- a/rtos/Mutex.h
+++ b/rtos/Mutex.h
@@ -135,16 +135,12 @@ public:
      */
     bool trylock_until(uint64_t millisec);
 
-    /** Unlock the mutex that has previously been locked by the same thread
-      @return status code that indicates the execution status of the function:
-              @a osOK the mutex has been released.
-              @a osErrorParameter internal error.
-              @a osErrorResource the mutex was not locked or the current thread wasn't the owner.
-              @a osErrorISR this function cannot be called from the interrupt service routine.
+    /**
+      Unlock the mutex that has previously been locked by the same thread
 
       @note You cannot call this function from ISR context.
      */
-    osStatus unlock();
+    void unlock();
 
     /** Get the owner the this mutex
       @return  the current owner of this mutex.