merge zoo

Dear zoo users,
I have used merge() succesfully to combine zoo objects uncountable times but
am running into a new issue.
When I call 
    merge(x1,x2)
Where x1 and x2 are two zoo objects I see the following error message:
    Error in z[match0(index(a), indexes), ] <- a[match0(indexes, index(a))]
: 
    number of items to replace is not a multiple of replacement length

I am quite stuck, any help would be greatly appreciated !

Thanks,
Markus