len [0, 1, 2] fails

[juanv]

    let
      x = [0, 1, 2]
      a = len([0, 1, 2]) # ok
      b = len @[0, 1, 2]  # ok
      c = len x # ok
      d = len [0, 1, 2] # fails
    
    
    Run

Why?