Loops with gmatch and multiple captures

This code:

``` lua
local function foo(s : string) end

local str = "Hello World"

for a : string, b : string in string.gmatch(str,"(%w)(%w)") do
	foo(a);
	foo(b);
end
```

Gives this error when compiling:

```
test.tl:7:2: type error, attempt to pass '(nil)' to local 'foo' of input type '(string, value*)'
 foo(b);
 ^
```