Oracle is a strange program

This program returns "OK" rather than "Error". After tracking, the program enters "return 'error'. But then it executes" return 'OK '.
Today, when I checked the program, I found a strange problem. As shown below.

Function chkA_ B return varchar2
A varchar2 (10): = NULL;
B varchar2 (10): = '1 ';
Begin
If a = B then
Return 'error ';
End if;
Return 'OK ';
End;

This program returns "OK" rather than "Error". After tracking, the program enters "return 'error'. But then it executes" return 'OK '.

If you change the if condition to if nvl (a, '0') = B, the program is normal.

The above strange program was tested on 9i. I wonder whether the subsequent version has changed or not, and I have not tested it.
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