getline(-1) returns 0

Can someone explain the following behavior?

The command

:echo getline(-1)

returns zero instead of an empty string. The corresponding help page states the
following.

> When {lnum} is a String that doesn't start with a digit, line() is called to
> translate the String into a Number.

> When {lnum} is smaller than 1 or bigger than the number of lines in the
> buffer, an empty string is returned.

Perhaps negative numbers are treated as strings and translated to zero by
line():

:echo line("-1")

returns zero. Using zero as an argument for getline(), the result should be an
empty string, as

:echo getline(0)

returns an empty string.

Thanks in advance.

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