df inside df not converted to JSON correctly

[SymbolixAU]

df <- structure(list(`_id` = 1, details = structure(list(
  val1 = "a", 
  val2 = "b", 
  val3 = "c", 
  val4 = "d", 
  val5 = "e"), 
  class = "data.frame", row.names = 1L)), class = "data.frame", row.names = 1L)

df
to_json( df )

Similarly

details <- data.frame(
  val1 = c("a")
  , val2 = c("b")
  , val3 = c("c")
  , stringsAsFactors = FALSE
)

df2 <- data.frame(
  id = 1
  , details = I(details)
)

And noting

details <- data.frame(
  val1 = c("a","b")
  , val2 = c("b","c")
  , val3 = c("c","d")
  , stringsAsFactors = FALSE
)

df2 <- data.frame(
  id = 1
  , details = I(details)
)

Doesn't print correctly, BUT, is a valid R structure (I think?)