python dictionary, assign random values the sum of which is a certain value

I am making a python program in which random values are generated n times, to be used as parameter values for model simulation.

I have a dictionary defining the boundaries for each parameter, for example:

parameters = {'A': random.uniform(1,10), 'B': random.uniform(20,40)}

I want to add some parameters the sum of which has to be 1, something like:

params = {'C1': random.uniform(0.0,1.0), 'C2': 1 - params['C1']}

This latter obviously doesn't work producing the KeyError: 'C1'

I also tried something like:

params = {'A': random.uniform(1,10), 'B': random.uniform(20,40), 'C': {'C1': None,'C2': None}}

def class_fractions():
    for key in params['C']:
        if key == 'C1':
            params['C'][key] = random.uniform(0.0,1.0)
        if key == 'C2':
            params['C'][key] = 1.0 - params['C'][key]

but after calling the function I get the TypeError

TypeError: unsupported operand type(s) for -: 'float' and 'NoneType'

Any suggestion?