syntax pb

I always have had difficulties to understand syntax. So...

If I have:

create table t1 (t text);

create table t2 (a text, b text, c test, d numeric);

insert into t1('azerty');

INSERT 0 1

fine !

so, now, if I write:

Select distinct test1.t, 'abc' as b, NULL as c, NULL as d 
From t1 test1;

   t      |  b   | c  | d
--------+-----+---+---
 azerty | abc |   |
(1 row)

ok.

and , now, if I want to insert that:

Insert into t2 (a, b, c, d)
Select distinct test1.t, 'abc' as b, NULL as c, NULL as d
From t1 test1;

I get:

ERROR:  column "d" is of type numeric but expression is of type text
LINE 2: Select distinct test1.t, 'abc' as b, NULL as c, NULL as d
                                                   
HINT:  You will need to rewrite or cast the expression.

Can someone give a short SQL syntax hint ?

thanks,