USB Serial Not working mbed os 6.1.0

Hi,

I have build USBSerial Example for mbed lpc1768 board.
But after flashing the code, usb is not enumerating.
When i tried debugging it i am getting following error on serial:

Error␣Status:␣0x80FF0144␣Code:␣324␣Module:␣255
Error␣Message:␣Assertion␣failed:␣status␣==␣osOK
Location:␣0x4295
File:␣.\mbed-os\rtos\source\ThisThread.cpp+225
Error␣Value:␣0x0
Current␣Thread:␣main␣Id:␣0x100014B4␣Entry:␣0x5679␣StackSize:␣0x1000␣StackMem:␣0x10001D98␣SP:␣0x10002CC4␣