How can it be possible the following calculation have negative values?

I am using the followind code :

 Output = bsxfun(@rdivide,(x*y'),z).*sqrt(k+n);

 size(x) = 1x1 
 size(y) = 1x50 
 size(z) = 50x36 
 size(k) = 50x36 
 size(n) = 50x36

All values in the matrices are positive. I cant figure out why 2 values get negative in the output.How could it be possible?

[dpb]

well, just to make sure let's see

all(y>0)
all(z(:)>0)
etc., ...

[John D'Errico]

You say the numbers are positive, but are they real numbers though? For example...

(1+sqrt(-1)) > 0
ans =
     1

When matlab does a compare with complex args here, it tests the real part, so it thinks that 1 + i is greater than 0.