cbind(NULL,zoo.object)?

Folks,

I often build up R objects starting from NULL and then repeatedly
using rbind() or cbind(). This yields code like:

a <- NULL
for () {
  onerow <- craft one more row
  a <- rbind(a, onerow)
}

This works because rbind() and cbind() are forgiving when presented
with a NULL arg: they act like nothing happened, and you get
all.equal(x,rbind(NULL,x)) or all.equal(x,cbind(NULL,x)).

This same idea is useful in building up a zoo matrix, by cbinding one
column after another. I find this quite elegant, though I'm conscious
that it leads to a lot of expensive memory management.

This approach breaks around zoo because cbind(NULL,z) doesn't work:

  > tmp <- structure(c(2.65917577210146, 1.17190441781228, 0.838363890267146, 
             -0.293979039853021, -0.132437820890186,
              0.262002985990861,  2.16601367541420, 
              0.375245019370496,  0.0108451048014047,
              1.56555812127923), index = structure(c(12509, 
              12510, 12513, 12514, 12515, 12516, 12521, 12524, 12528,
              12529), class = "Date"), class = "zoo")
  > tmp
   2004-04-01  2004-04-02  2004-04-05  2004-04-06  2004-04-07  2004-04-08 
   2.65917577  1.17190442  0.83836389 -0.29397904 -0.13243782  0.26200299 
   2004-04-13  2004-04-16  2004-04-20  2004-04-21 
   2.16601368  0.37524502  0.01084510  1.56555812 
  > cbind(NULL,tmp)
        12509       12510       12513       12514       12515       12516 
   2.65917577  1.17190442  0.83836389 -0.29397904 -0.13243782  0.26200299 
        12521       12524       12528       12529 
   2.16601368  0.37524502  0.01084510  1.56555812 
  Warning message:
  In merge.zoo(..., all = all, fill = fill, suffixes = suffixes, retclass = "zoo") :
    Index vectors are of different classes: integer Date

Is there an easy workaround; or am I deeply confused; is there a way
out? :-)