Using data not sorted (nearmrg)

Hello,

I am trying to use nearmrg on my data files and I keep getting the same error: "Using data not sorted". As I wanted to break down the problem I used very simple test data instead of my real data and the error message still shows up. Now I have the following:

Master.dta
Group Date
A 15.01.2012
A 15.02.2012
B 15.01.2012
B 15.02.2012
C 15.01.2012
C 15.02.2012

Using.dta
Group Date SVarOfInterest1 SVarOfInterest2
A 01.01.2012 1 201
A 15.01.2012 2 202
A 03.02.2012 3 203
A 23.02.2012 4 204
B 03.01.2012 11 211
B 19.01.2012 12 212
B 03.02.2012 13 213
C 20.01.2012 21 221
C 25.01.2012 22 222
C 04.02.2012 23 223
C 03.01.2012 24 224

This is the code:

nearmrg Group using Using.dta, nearvar(Date) genmatch(SourceDate) lower
using data not sorted
r(5);

Thanks in advance!
Maryna