FILE INDEX ERROR 20

This report is given when an attempt is made to reference a non-existent component.

Example

      ⎕FSIZE 1
1 21 16578 4294967295

      ⎕FREAD 1 34
FILE INDEX ERROR
      ⎕FREAD 1 34
      ^
      ⎕FDROP 1 50
FILE INDEX ERROR
      ⎕FDROP 1 50
      ^