col_plan with span_structure does not work if package not attached

[bzkrouse]

works:

col_plan( 
  span_structure(
    c1 = "Top Label Level 1" 
  ))

does not work:

tfrmt::col_plan(
  tfrmt::span_structure(
    c1 = "Top Label Level 1"
  )
)

The following error is returned:

Error in if (as.character(as.list(x)[[1]]) == "-") { : 
  the condition has length > 1