Plotting a symbolic double integral with ezcontour

I'm getting familiar with the symbolic matlab package, and want to run a parameter sweep in two dimensions, siginflec and leak, and get a contour plot of the results

syms t t0 tpast T siginflec leak reward x y;
% deriv of function at time t0
% *needs params {siginflec, t0} to return a double
fprime = 1. / (1. + exp(-siginflec*t0));

% determines the temporal-discounted-running average of the deriv
% *needs params {siginflec, leak, t} to return a double
leakyderiv = int(exp(-leak*(t-tpast)) * subs(fprime, t0, tpast),...
                 tpast, [0, t]);

% integral of the above term across an episode
% *needs params {siginflec, leak, T}
evleakyderiv = int(leakyderiv, t, [0, T]);

I've confirmed that when I run double(subs(evleakyderiv, {siginflec, leak, T}, {1., 1., 1.})), I get a value (0.2161 if you want to validate).

However, when I try to run

ezcontour(subs(evleakyderiv, {T}, {1.}), [0., 100.], 101)

I get the following error:

Undefined function or variable 't'.

Error in symengine>makeFhandle/@(x,y)int(int(exp(-y.*(t-tpast))./(exp(-tpast.*x)+1.0),tpast,0.0,t),t,0.0,1.0)

I've tried replacing siginflec and leak with x and y to see if that makes it play nice with ezcontour, as well as

ezcontour(subs(evleakyderiv, {siginflec, leak, T}, {x, y, 1.}), [0., 100.], 101)

to see if the vars needed to be passed directly to the function, but no go. Can someone help me what I'm doing wrong? I'm going off the the example here.