Problem with MuPad assumptions

Consider the following lines of code.

 assume(h > 0)
assume (`ν` > 0)
assume (U > 0)
Pe := U * h / (2 * `ν`)
`&xi` := cosh(Pe)/sinh(Pe) - 1 / Pe
is(`&xi` >= 0

'Pe' should be positive, and when 'Pe' is positive, the variable `ξ' is always positive. But I run these lines in Mupad, the answer comes out to be

UNKNOWN

What is the problem here? I have plotted the variable `ξ' against Pe from 0 to 100000, and `ξ' is always positive.

Is it possible to set the force the sign of `ξ' to be positive?