Getting error while initializing the array variable in fast formula

Summary

I am getting the error while initializing the variable "PER_EXT_GRADE_STEP_RATE_VALUE" as its data type is "Text value & number index array" It is erroring out at the DEFAULT section itself.

Content

/*==== Formula to pick current step rate ======*/
/*=========== DATABASE ITEM DEFAULTS BEGIN ==============*/
DEFAULT FOR PER_EXT_GRADE_STEP_RATE_VALUE IS ''
DEFAULT FOR L_DEFAULT_VALUE IS '1'
/*=========== DATABASE ITEM DEFAULTS END ================*/
/*=========== INPUT SECTION BEGIN ==============*/
INPUTS ARE CMP_IV_PLAN_ID (number)
/*=========== INPUT SECTION ENDS ==============*/
/*=========== FORMULA SECTION BEGIN =====================*/
CHANGE_CONTEXTS(LEGISLATIVE_DATA_GROUP_ID=300000001091110)
(
L_DEFAULT_VALUE = PER_EXT_GRADE_STEP_RATE_VALUE
)
RETURN L_DEFAULT_VALUE