Melting a cast data frame gives incorrect output (Hadley Wickham's reshape package in R)

Question

I've encountered a strange behaviour in cast/melt from Hadley Wickham's reshape package. If I cast a data frame, and then try to melt it, the melt comes out wrong. Manually unsetting the "df.melt" class from the cast dataframe lets it be melted properly.

Does anyone know if this is intended behaviour, and if so, what is the use case when you'd want it?

A small code example which shows the behaviour:

> df <- data.frame(type=c(1, 1, 2, 2, 3, 3), variable="n", value=c(71, 72, 68, 80, 21, 20))

> df
  type variable value
1    1        n    71
2    1        n    72
3    2        n    68
4    2        n    80
5    3        n    21
6    3        n    20

> df.cast <- cast(df, type~., sum)
> names(df.cast)[2] <- "n"

> df.cast
  type   n
1    1 143
2    2 148
3    3  41

> class(df.cast)
[1] "cast_df"    "data.frame"

> melt(df.cast, id="type", measure="n")
         type value value
X.all.      1   143 (all)
X.all..1    2   148 (all)
X.all..2    3    41 (all)

> class(df.cast) <- "data.frame"
> class(df.cast)
[1] "data.frame"

> melt(df.cast, id="type", measure="n")
  type variable value
1    1        n   143
2    2        n   148
3    3        n    41