Estoy tratando de fundir un marco de datos y recibo este error extraño. ¿Alguna idea de por qué?

str(zx7)
'data.frame':   519 obs. of  5 variables:
 $ calday.new: Date, format: "2011-01-03" "2011-01-04" "2011-01-05" "2011-01-06" ...
 $ A20    : Time-Series  from 1 to 519: 0 0 0 0 0 0 0 0 0 0 ...
 $ B20    : Time-Series  from 1 to 519: 0 0 0 0 0 0 0 0 0 0 ...
 $ C20    : Time-Series  from 1 to 519: 0 0 0 0 0 0 0 0 0 0 ...
 $ D20    : Time-Series  from 1 to 519: 0 0 0 0 0 0 0 0 0 0 ...

zx7.melt <- melt(zx7, id=c("calday.new"))
Error in `[<-.ts`(`*tmp*`, ri, value = c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  : only replacement of elements is allowed