MM using AMBER forcefield

[gaia di paolo]

Dear All,
i'm trying to perform a MM run using AMBER forcefield, but i'm getting
the following error:

------------------------------------------------------------------------------------------------------------------------------
 *** ERROR in check_subsys_element  ***
 *


 ***  Unknown element for KIND <CX2>. This problem can be fixed
specifying  ***
 *** properly elements in PDB or specifying a KIND section or getting
in    ***
 *** touch with one of  the
developers!                                     ***


 ===== Routine Calling Stack =====

            6 connectivity_control_check_element_name
            5 connectivity_control
            4 topology_control
            3 fist_init
            2 fist_create_force_env
            1 CP2K
 CP2K| Abnormal program termination, stopped by process number 0
------------------------------------------------------------------------------------------------------------------------------



Actually i don't have any KIND CX2 in my forcefield.
What 's the meaning?

Thank you in advance
Best regards
Gaia