(module
 (type $v (func))
 (import "env" "fn" (func $fn))
 (memory $0 0)
 (export "main" (func $main))
 (func $main (; 1 ;) (type $v)
  (call $fn)
 )
)

[wasm-validator error in function $main] unexpected false: call target must exist, on 
[none] (call $fn)
(perhaps it should be a CallImport instead of Call?)