Careful measurements and calculations of the neutron spectrum in an aqueous solution of erbium nitrate are described. The validity of the infinite medium calculation is scrutinizingly studied. As a result it is shown that the discrepancy between measurement and calculation is caused by a too high (by about 10%) absorption cross section of erbium. This result is substantiated by two independent integral tests; the fundamental time eigenvalue of the system and the neutron conservation test.