Hi I want to know how to test if the sum of two parameters are zero in the mlogit context.

<Notations>
y: the nominal dependent variable having three values, 1, 2, and 3
x1, x2: the unit specific regressors (They are alternative-invariant. Thus, we have two parameters for each regressor)
d1: a dummy variable

<Code>
Code:
mlogit y x1 x2 i.d1#c.x1
In this situation, I am wondering how to test the sum of the coefficients of x1 and i.d1#c.x1.

I tried to the following commands.
Code:
testparm x1 i.d1#c.x1, equal
       Constraint 1 dropped

           chi2(  0) =       .
         Prob > chi2 =         .

testparm [2]x1 [2]i.d1#c.x1, equal
varlist required
In addition to the above commands, I tried many other command but I was not able to run the test.
How can I test if the sum of two parameters are zero in multinomial logit context?

Thank you for your time spent to read this question.