Unclear syntax error message for block of code

[KristofferC]

Pasting this:

D["a"] = function (s,o...)
            if a == 0
                f(a, b) do
                end
            end
        end,
D["b"] = function (s,o...)
    if  b == 0
        f(a, b) do
        end
    end
end

gives:

ERROR: syntax: invalid assignment location "(function (tuple s (... o)) (block (line 2 REPL[17]) (if (call == a 0) (block (line 3 REPL[17]) (call f (-> (tuple) (block (line 4 REPL[17]))) a b)))))"