PowerConnect 6248 problem with dot1dStpPortState(1.3.6.1.2.1.17.2.15.1.3)

Hello everyone, Could you please help me with this problem? I use PHP5 to get information about switch(PowerConnect 6248). It works fine. But when I want to get information about dot1dStpPortState I receive error message from PHP5 :

 "Warning: snmpget(): Error in packet: (noSuchName) There is no such variable name in this MIB. in C:\Program Files\Apache Software Foundation\Apache2.2\htdocs\mapa\index.php on line 1973 Warning: snmpget(): This name does not exist: iso.3.6.1.2.1.17.2.15.1.3.43 in C:\Program Files\Apache Software Foundation\Apache2.2\htdocs\mapa\index.php on line 1973"

I also tried OpManager to chech MIB tables. And i receive similar error. It looks like tree iso.3.6.1.2.1.17.2.15.1 doesnt work. Question is if someone have similar problem and how he/she solve it. Or I am doing something wrong. Thx you for your help.