指数积分函数和级数求和的结合，错在哪里了

>> syms m
f1=expint(-0.004/(4e-6*(1800-m*1800-1440)));
f2=expint(-0.004/(4e-6*(1800-m*1800)));
f3=f1-f2;
T=symsum(f3,m,0,inf)
??? Inputs to superiorfloat must be floats, namely single or double.

Error in ==> expint at 35
y = zeros(numel(x),1,superiorfloat(x));