SQL-2003 (ISO/IEC 9075-2:2003 (E)):
6.11 <case expression>
[...]
<case abbreviation> ::=
NULLIF <left paren> <value expression> <comma> <value expression> <right paren>
| COALESCE <left paren> <value expression> { <comma> <value expression> }... <right paren>
[...]
I.e. two arguments or more.