I bulid a custom indicator and try to warm it up.

This should initate the warm up: self.slopy[symbol].Warmup(history)

I commented it out in the code as it gives the following error:

During the algorithm initialization, the following exception has occurred: TypeError : 'custom_slope' object is not subscriptable
at Initialize in main.py:line 20
:: self.slopy[symbol].Warmup(history)
TypeError : 'custom_slope' object is not subscriptable

The custom indicator is very similar from this discussion:

https://www.quantconnect.com/forum/discussion/9976/custom-indicators-history-in-alpha-framework/p1

What do I need to change?