How to resolve __debug__ error?

I'm using Keil tools. I get following link errors:

.\Builds\Octopus47.axf: Error: L6218E: Undefined symbol __debug__ (referred from at_ip.o).
.\Builds\Octopus47.axf: Error: L6218E: Undefined symbol __error__ (referred from at_ip.o).

How can they be resolved? Is _ip.o Keil's TCP Stack library file?

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