uncertain prameters (robust control design)

Hi everyone, I have two uncertain parameters m and k, which i have declared using

m=ureal('m',density*vol,'Percentage',10)

k=ureal('k',20,'Percentage',10)

However, later for my system matrix A, one of the element is square root of the product of m and k. Matlab gives an error message when finding square root of uncertain parameters. Is there any alternative way to find a square root of uncertain parameters? Any suggestions will be much appreciated. thanks