Question: invalid option from rsolve in SumTools

with(SumTools):
rsolve({c*(n+beta)*P(n+1)/(-1+c)+(c*(x+beta)/(1-c)-x)*P(n)+n*P(n-1)/(-1+c)=0,P(0)=1}, P, 'genfunc'(z));

all given, but rsolve said invalid options, why P not in terms of other variables?

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