Cannot use solver function with poisscdf function

syms xu
eqn = 0.05 == 1 - poisscdf(35,xu);
S = solve(eqn,xu)

I want to get the mean of the poisson distribution

However, when I run the matlab code above I get the below error

% Error using symengine
% Unable to prove 'xu < 0' literally. Use 'isAlways' to test the statement mathematically.
% Error in sym>logicalNaNIsFalse (line 1965)
% X = mupadmex('symobj::logicalNaNIsFalse',A.s,9);
% Error in sym/any (line 603)
% X = any(logicalNaNIsFalse(A));
% Error in poisscdf (line 45)
% if any(t(:))

Answer 1

[Torsten]

syms lambda n
eqn = exp(-lambda)*symsum(lambda^n/factorial(n),n,0,35) == 0.95;
solve(eqn,lambda)

Warning: Unable to solve symbolically. Returning a numeric solution using vpasolve.

ans = 

26.73116648164810130021030790587