Why won't nested select-into expression work?

I've got an expression that works fine if I assign its return value (bigint) to a temporary variable (t).  But if
I eliminate the variable and just nest the expression, its outer expression (select into) fails for some reason.

Relevant variables:
    my_paragraph    paragraph%rowtype;
    t bigint;

I tried this and it failed (see embedded comments):

        select * into my_paragraph from paragraph where
            form = p_form and
            number = ( select method_paragraph_new( p_form, 0, p_append ) );
            -- Inner expression above inserts a new row in the 'paragraph' table which
            -- just happens to be the one I want selected by the outer select-into expression
        if not found then
            raise exception 'DEBUG: Paragraph create failed';  -- This is what happens!
        else
            raise exception'DEBUG:  Success!';
        end if;

But it works just fine if I use the variable 't' instead:

        t := ( select method_paragraph_new( p_form, 0, p_append ) );
        select * into my_paragraph from paragraph where
            form = p_form and
            number = t;  -- instead of a nested expression that inserts the thing I am looking for
        if not found then
            raise exception 'DEBUG: Paragraph create failed';
        else
            raise exception'DEBUG:  Success!';  -- This is what happens!
        end if;

There's probably something fundamental I am not understanding here.  Does anyone see the problem?