diff --git a/rtos/include/rtos/Thread.h b/rtos/include/rtos/Thread.h
index 1aa72de..2083e32 100644
--- a/rtos/include/rtos/Thread.h
+++ b/rtos/include/rtos/Thread.h
@@ -131,7 +131,8 @@
 
     /** Starts a thread executing the specified function.
       @param   task           function to be executed by this thread.
-      @return  status code that indicates the execution status of the function.
+      @return  status code that indicates the execution status of the function,
+               or osErrorNoMemory if stack allocation failed.
       @note a thread can only be started once
 
       @note You cannot call this function ISR context.
diff --git a/rtos/source/Thread.cpp b/rtos/source/Thread.cpp
index 8d1fa92..3b9a8d9 100644
--- a/rtos/source/Thread.cpp
+++ b/rtos/source/Thread.cpp
@@ -81,8 +81,11 @@
     }
 
     if (_attr.stack_mem == nullptr) {
-        _attr.stack_mem = new uint32_t[_attr.stack_size / sizeof(uint32_t)];
-        MBED_ASSERT(_attr.stack_mem != nullptr);
+        _attr.stack_mem = new (std::nothrow) uint32_t[_attr.stack_size / sizeof(uint32_t)];
+        if (_attr.stack_mem == nullptr) {
+            _mutex.unlock();
+            return osErrorNoMemory;
+        }
     }
 
     //Fill the stack with a magic word for maximum usage checking