Json: Error: type mismatch: got '...' but expected '(string, string)'

[scippie]

I am very new to the Nim language. However, in only 2 hours time, I 
successfully converted an existing project of mine into Nim. Wow! However, now 
I want to be able to store/restore an object by using a json file.

When I try the code at the nim-by-example json tutorial, it works perfectly.

However, when I try this code (in a separate module):
    
    
    import std/json
    
    type
      Planet* = ref object
        name*: string
        pos*: tuple[x: float, y: float]
    
    proc `%*`(planet: Planet) : JsonNode =
      result = %* {
        "name": planet.name,
        "pos": [ planet.pos.x, planet.pos.y ]
        }
    
    
    Run

I get this error:
    
    
    Error: type mismatch: got '(string, array[0..1, float])' for '("pos", 
[planet.pos.x, planet.pos.y])' but expected '(string, string)'
    
    
    Run

Why am I getting this error? I don't see the difference with the tutorial where 
an integer is put into the json data. I changed it to a tuple and it works too.