Why string can be added with 0?

[Peng Yu]

I see this. I don't find anything about it in 6.2.1 Arithmetic Operators.

$ gawk '{ print typeof($1), $1 + 0 }' <<< a
string 0

But it seems that there should be an error to add a string to 0? Is it
better to show some error instead of assuming a string as 0 in the
context of arithmetic operations? Thanks.

-- 
Regards,
Peng