ambigous method error on missings

The following code converts from an Vector{Tuple} to a Vector of first entry of the Tuple. I expected that I can access it like a Vector. However, currently it errors with 'method is ambiguous'. How can I make it work?

```
tvec = allowmissing([(rand(),rand()) for i=1:6]);
tvec[1] = missing
c = mappedarray(x-> ismissing(x) ? missing : first(x),tvec);
c[1:2]
```