Hi. It's default window:
window = fspecial('gaussian', 11, 1.5);
If input images has smaller size than 11*11 (it's window size), the output became -Inf. You may change default window:
a=[191 161;198 197]
b=[201 191; 166 195]
K(1) = 0.01; % default settings
K(2) = 0.03; %
window = ones(2);
mssim = ssim(a,b, K, window);
