Step-Loops

Hi,

I want to display dynamic material numbers on a custom screem based on input but i am getting the following Dump:

Runtime Errors         DYNPRO_ITAB_ERROR
Date and Time          12/23/2012 08:40:33

Short text
     Error in internal table in screen

What happened?
     The current screen processing action was terminated since a situation
     occurred where the application could not continue.
     This is probably due to an error in the ABAP program or in the current
     screen.

Error analysis
     The program "YTESTFORM" has terminated.

     The following error occurred (short text of cause of error):
     "FX099: Wrong fieldtype for CURSOR. Has to be binary/hex type."

     The following elements are affected:
     Screen name.............. "YTESTFORM"
     Screen number............ 0200

     This error is connected to the processing of an internal table with the
     field or variable "C", and was triggered within in the program.

Trigger Location of Runtime Error
     Program                                 YTESTFORM
     Module Name                             SYSTEM-EXIT

Code below:

PROCESS BEFORE OUTPUT.
   loop at i_ytest2 into w_ytest2 cursor c from n1 to
    n2.    (Getting dump on this loop)
    chain.
      field: ytest11-matnr.
      module dynpro_modifiz_loop_0200.
    endchain.
  endloop.
*
PROCESS AFTER INPUT.

  loop.
    chain.
      field: ytest11-matnr.
*      module dynpro_0901_input.
    endchain.
  endloop.