SUM function and missing value

[William_Anthem]

Can anyone please clarify the following for me?  I have two different answers from certification practice exams and I really don't understand.  Example 2 makes sense to me...SUM ignores missing values.  But then there's Example 1 that uses SUM but in that one "missing" makes the total SUM missing.  Any clarification is greatly appreciated - thanks!

Example 1

ACredit = . ;
BCredit = 1;
TotalCredits = sum (ACredit + BCredit) + 2;
    What is the value of TotalCredits?  Answer = Missing numeric value
    Explanation: when the SUM function is called with a single argument, it returns the value of that argument. In this case, it is called    
    with a missing value, so it returns a missing value.

Example 2
ACredit = . ;
BCredit = 1;
TotalCredits = sum (ACredit, BCredit) + 2;
    What is the value of TotalCredits?  Answer = 3
    Explanation: The SUM statement ignores missing values.

[Reeza · Accepted solution]

The ACredit+Bcredit is evaluated first inside the brackets and returns a missing value to SUM() which then returns a missing value again.

[William_Anthem]

Thank you Reeza, that explains it very well! 

I appreciate your help!

Bill