Calling cocotb_bus BitDriver.start() causes an exception error

[chooob-bot]

cocotb version 1.8.1
Linux 64 bits
Questa 10.7c
Python 3.6

@cocotb_test
async def my_test(dut)
driver=BitDriver(dut.clk, dut.data_in)
cocotb.fork(Clock(dut.clk, CLK_PERIOD, units='ns').start())
driver.value = 1 driver.start()
await ClockCycles(dut.clk,100)

Error - BitDriver._cr_twiddler Exception raised by this forked cocoroutine
traceback:
scheduler.py, line 384 in _react self._event_loop(trigger)
scheduler.py, line 487 in _event_loop self._schedule(coro, trigger = trigger)
scheduler.py, line 956 in _schedule Assert result is none
Assertion error

Note that even if the only code in the test is just
driver = BitDriver(dut.clk, dut.data_in)
driver.start()

It still fails on that exception