fill 0-row data.frame with 1 line of NAs

Dear all
Is there a simpler method to achieve the following: When I obtain an
empty data.frame after subsetting, I need for it to contain one line
of NAs. Here's a dummy example:

(.xb <- iris[ iris$Species=='zz', ])

[1] Sepal.Length Sepal.Width  Petal.Length Petal.Width  Species
<0 rows> (or 0-length row.names)

dim(.xb)

[1] 0 5

(.xa <- data.frame(matrix(rep(NA, ncol(.xb)), 1)))

   X1 X2 X3 X4 X5
1 NA NA NA NA NA

names(.xa) <- names(.xb)
(.xb <- .xa)

   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1           NA          NA           NA          NA      NA


The solution I came up with is way too convoluted. Anything simpler? Regards
Liviu