SNMP issue: Evaluation failed for function: delta

[supertek]

Hello,
I have an issue with all my host using snmp template. I receive datas from all hosts, but I have some items unknown, when I go check on the logs I get a lot of those messages:

966:20110727:145624.602 Expression [{13163}>150000] cannot be evaluated: Evaluation failed for function: delta
968:20110727:145624.610 Evaluation failed for function: delta
968:20110727:145624.610 Expression [{14421}>0] cannot be evaluated: Evaluation failed for function: delta
966:20110727:145624.610 Evaluation failed for function: delta
966:20110727:145624.611 Expression [{13168}>0] cannot be evaluated: Evaluation failed for function: delta
966:20110727:145624.613 Evaluation failed for function: delta

Can you tell me what could be the problem? I am running zabbix 1.8.3.

[Alexei]

These messages can be ignored. I believe they do not appear in the log file starting from version 1.8.5.

[supertek]

Ok, but I get those messages because of something is wrong with my hosts checked by snmp, right?
For example, I check a router with the snmp template, in 207 triggers, I receive 44 unknowns, when i go check what is wrong on the items i get this message: Evaluation failed for function: delta.
So what can I do? What can cause that?