如何使reshape2 :: acast在变量聚合时抛出错误？

> DF = data.frame(expand.grid(LETTERS[1:3], LETTERS[4:5])[-3,], value=1:5)
> DFa = rbind(DF, c("A","D",6))
> DF
  Var1 Var2 value
1    A    D     1
2    B    D     2
4    A    E     3
5    B    E     4
6    C    E     5
> DFa
   Var1 Var2 value
1     A    D     1
2     B    D     2
4     A    E     3
5     B    E     4
6     C    E     5
61    A    D     6
> DF = data.frame(expand.grid(LETTERS[1:3], LET