HPCC-12380 Remove internal error (triggered by invalid source)

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6522 from ghalliday/issue12380

HPCC-12380 Remove internal error (triggered by invalid source)

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>