diff --git a/rtos/rtos/Semaphore.h b/rtos/rtos/Semaphore.h
index b4d294256a..aeb4636c2f 100644
--- a/rtos/rtos/Semaphore.h
+++ b/rtos/rtos/Semaphore.h
@@ -31,9 +31,9 @@ namespace rtos {
 class Semaphore {
 public:
     /** Create and Initialize a Semaphore object used for managing resources.
-      @param number of available resources; maximum index value is (count-1).
+      @param number of available resources; maximum index value is (count-1). (default: 0).
     */
-    Semaphore(int32_t count);
+    Semaphore(int32_t count=0);
 
     /** Wait until a Semaphore resource becomes available.
       @param   millisec  timeout value or 0 in case of no time-out. (default: osWaitForever).