function to_char(unknown, unknown) is not unique

Hello.

When I'm trying to:
select to_char("ISDate", 'DD.MM.YYYY')::text from register."KSPT_view"
from table, containing nulls in "ISDate", i get nulls and formatted date as
output.

But when I'm trying to:
select to_char(null, 'DD.MM.YYYY')::text

I get a error message:

ERROR: function to_char(unknown, unknown) is not unique
LINE 1: select to_char(null, 'DD.MM.YYYY')::text
^
HINT: Could not choose a best candidate function. You might need to add
explicit type casts.

********** Error **********

ERROR: function to_char(unknown, unknown) is not unique
SQL state: 42725
Hint: Could not choose a best candidate function. You might need to add
explicit type casts.
Character: 8