Passing a dynamic interval to generate_series()

Hello,

I am trying to pass a dynamic interval to generate_series() with date range.

This works as expected, and generates a series with an interval of 1 month:

SELECT generate_series(
date_trunc('month', current_date),
date_trunc('month', current_date + interval '7 month'),
interval '1 month'
)

This works as expected and returns an interval of 1 month:

SELECT ('1 ' || 'month')::interval;

But this throws an error (SQL Error [42601]: ERROR: syntax error at or near
"'1 '"):

SELECT generate_series(
date_trunc('month', current_date),
date_trunc('month', current_date + interval '7 month'),
interval ('1 ' || 'month')::interval
)

And this returns a series with interval of 1 second??

SELECT generate_series(
date_trunc('month', current_date),
date_trunc('month', current_date + interval '7 month'),
(interval '1 ' || 'month')::interval
)

Because this returns an interval of 1 second:

SELECT (interval '1 ' || 'month')::interval;

Is that a bug?

I am able to work around the issue using a CASE statement, but shouldn't it
work simply by concatenating the string with the || operator?

Thank you,

Igal