Killing Tags with MC9090/RFID


I want to kill a Class1Gen2 tag using RFID2.Device: KillTagID method (C# EMDK2.4 / 2.5) on an MC9090/RFID. I always get an exception "Error in locking tags". I also tried the CS_RFID2_Sample and the example code in the help file and got the same result. I tried several different tag types. I get the same exception when no tag is in the area. 

What am I doing wrong?