Postgres retruns zero record (not DB-Null) by MAX()?

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C#

PostgreSQL

Someone please help me.

I'm not good in English, so my explanation might be ununderstandable a little.
Sorry about that.

Main question is
"Would MAX() return zero record?"

Following is my situation.

My application -coded in c#- connects Postgres with Npgsql.

I've understood MAX() as function which never returns zero record*.
However, it retruned zero record when application issued a SQL which gets MAX() by ExecuteScalar.

Officials say that ExecuteScalar returns
"The first column of the first row in the result set, or a null reference (Nothing in Visual Basic) if the result set is empty."
And in my case, returned object is Null.
If so, I think MAX() would return zero record in a certain case.

Then, go back to the question
"Would MAX() return zero record?"
If the answer is Yes,
"when is it??"
If the answer is No,
"why was the object Null??"

I'm having trouble reproducing this phenomenon for a long time.
Please give me your hand.


*object is primarykey.