How to define a dispersive material as a function of COMSOL predefined frequency in emw interface?

Hi:

I defined a dispersive material function (frequency dependent relative permittivity) in Global Definition.

Function name: LD_real
Expression: ff E-14+0.6
Argument: ff
Argument unit: Hz
Function unit: 1

Then I assigned Relative Permittivity (real part) to a material defined in the Material tab with this value: LD_real(emw.freq)

However, when I started to compute the system error message said "Undefined value found¨.
I specified the relative permittivity with LD_real(1.2 E14), there is no error message appeared.
Sso obviously the solver cannot recognize emw.freq, why??
emw.freq is COMSOL predefined variable, why the system cannot take it?

I was using eigenvalue solver for eigen frequency study.
Because the material is frequency dependent, the solver must be able to change the material value automatically every time when finding another eigen frequency.
so how can I solve this problem?