You can only read out of the LRAW or LCHR field CUKN~KNBLK using its prepending length field

hi guys,
in one of the report i have written the following query:

DATA: LV_KNBLK TYPE CUKN-KNBLK,
      LINE_CNT(6).
  CLEAR: LV_KNBLK, LINE_CNT,  WA_CUKN.

    SELECT SINGLE CUKN~KNBLK INTO LV_KNBLK
      FROM CUKN INNER JOIN CUKB
      ON   CUKB~MANDT EQ CUKN~MANDT AND CUKB~KNNUM EQ CUKN~KNNUM
      WHERE CUKB~KNNAM EQ WA_ITAB-KNNAM.
and it was giving a dump

error analysis was having the following statement:

"You can only read out of the LRAW or LCHR field CUKN~KNBLK using its prepending length field"

then i searched on SCN and found the solution of it, then i modified the query as mentioned below and now report is running fine.

DATA: LV_KNBLK TYPE CUKN-KNBLK,
      LINE_CNT(6).

    CLEAR: LV_KNBLK, LINE_CNT,  WA_CUKN.
    SELECT SINGLE * INTO CORRESPONDING FIELDS OF WA_CUKN
      ON   CUKB~MANDT EQ CUKN~MANDT AND CUKB~KNNUM EQ CUKN~KNNUM
      WHERE CUKB~KNNAM EQ WA_ITAB-KNNAM.

But still i am unaware of this error analysis : specifically "prepending length field" term

"You can only read out of the LRAW or LCHR field CUKN~KNBLK using its prepending length field"

can you please make understand what exactly is this, when we get such errors??

why i was not able to fetch that single column from the table to the variable??

thank you!!