I use the following code to calculate maximum eigenvalue in stata

encode Name, gen(name)
spset name, coord(latitude longitude )
spset, modify  coordsys(latlong)

spmat idistance W longitude latitude,id(name) replace
spmat getmatrix W mataW
spmat eigenvalues W
spmat getmatrix W, eig(lambda)

mata:
        mataW_n = mataW/max(lambda)
end

But I got the following error:

mataW_n = mataW/max(lambda)
                minmax():  3253  nonreal found where real required
                   max():     -  function returned error
                 <istmt>:     -  function returned error
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I have tried mata function of eigensystem() to get the eigenvalue L, and then use max(L). But it got the same error. How to deal with this problem?

Many thanks!