interpstreamspeed problem, only getting 1 pair of values from each 10000 pair cell

I'm having a problem with interpstreamspeed and I can't figure out what it is.

I have two sets of vertices, both calculated from the same set of original data. When I use interpstreamspeed on one set, everything is fine, but when I do it on the other, I only get 1 set of values for each vertice and I'm not sure why.

Relevant bits of program

[x y] = meshgrid(-4.55:0.02:4.61, -4.55:0.02:4.61);

% data for positive charge at (-1,0) and negative charge at (1,0)
V1 = 1./sqrt((x+1).^2+y.^2);
V2 = -1./sqrt((x-1).^2+y.^2);
V = V1+V2;

[Ex, Ey] = gradient(-V,0.02,0.02);

% first set of vertices start from a small circle around charge 1
[theta r] = meshgrid(0:pi/12:2*pi, 0.01);
[startx starty] = pol2cart(theta, r);

verts = stream2(x,y,Ex,Ey,startx-1,starty);

% second set of vertices fill in gaps to right of charge 2
for n = 1:length(verts)
    if verts{1,n}(end,1)<startx
        startx2 = [startx2 verts{1,n}(end,1)];
        starty2 = [starty2 verts{1,n}(end,2)];
    end
end

verts2 = stream2(x,y,Ex,Ey,-startx2,starty2);

% problem part, interpstreamspeed gives me two cell arrays, but the one for iverts2 contains many pairs of xy data in each cell
% iverts only contains the first xy pair in verts in each cell
iverts = interpstreamspeed(x,y,Ex,Ey,verts,0.1);
iverts2 = interpstreamspeed(x,y,Ex,Ey,verts2,0.1);

Any help would be much appreciated