[R] aggregate() - error message

[Juliane Struve]

Dear list,
 
would anybody be able to tell me why the statement 
 
Tripstatistics=aggregate(TripsData[2:3],by=list(Trip=Tripmatch),FUN="mean")
 
seems to work well with TripsData 1 but not with TripsData 2 ?
 
With TripsData 2 it yields 
 
Error in FUN(X[[1L]], ...) : arguments must have same length

I can't see a difference in the two data sets. Could someone shed light on the error message ? Maybe that would help me figure out where the problem is.
 
Thanks very much for any help. 
 
Juliane 
 
PS: The examples below contain the  first 5 rows  of my two datafiles.  
 
 
TripsData 1 

           Trip Distance TimeDiff Tripmatch
7329 35,36 172.7453   0.0041     35,36
7371 36,35 172.7453   0.0004     35,36
7372 35,36 172.7453   0.0000     35,36
7373 36,35 172.7453   0.0004     35,36
7374 35,36 172.7453   0.0001     35,36

 
TripsData 2
 
            Trip Distance TimeDiff Tripmatch
1617 19,20 1087.365   0.0441     19,20
1899 20,19 1087.365   0.0207     19,20
1915 19,20 1087.365   0.0361     19,20
3285 20,19 1087.365   0.0356     19,20
3826 19,20 1087.365   0.0697     19,20