How do i implement a sum containing the index as an exponent and plot it?

Hey there,

I am trying to use the following function in Matlab:

I've tried using the symsum function to implement my problem but I receive errors. This is the script I'm trying to run:

clear all
close all
clc

n_k=291;

T=0:0.01:1;

syms u;
expr=(n_k*T^(u-1))/factorial(u-1);
F_kt(T)=1- exp(-n_k*T)*symsum(expr, u, 1, n_k);

plot(T,F_kt)

I receive the following errors:

Error using mupadmex
Error in MuPAD command: Not a square matrix.
[(Dom::Matrix(Dom::ExpressionField()))::_power]

Error in sym/symsum (line 65)
   rSym = mupadmex('symobj::map',f.s,'symobj::symsum',x.s,a.s,b.s);

Error in TEST (line 15)
F_kt(T)=1- exp(-n_k*T)*symsum(expr, u, 1, n_k);

As far as I understand there is a problem in the exponent of T. I'd be grateful if anyone could explain to me what I am doing wrong.

Regards