why this is shift/reduce?

[Jibin Han]

Hello,
    Bison complains the following shift/reduce error at
chip_param_define section(the last part), I am confused here, could you
tell me why it is?

board:chips {}

chips: chip {}
  |
  chips chip {}

chip: BEGIN_FPGA chip_defines cores END {}

chip_defines: chip_param_define {}
  |
  chip_defines chip_param_define {}

chip_param_define: /* empty */
  |
  PARAMETER_INSTANCE  CHIP_INSTANCE_NAME {}
  |
  DEVICE CHIP_DEVICE_NAME {}