A 76-year-old Afro-Caribbean man presenting with heart failure was diagnosed with isolated cardiac amyloid. He had evidence of myeloma on bone marrow biopsy suggesting AL amyloid, the commonest type of systemic amyloidosis, as the underlying cause. He had no other myeloma-related organ damage. However, endocardial biopsy revealed amyloid fibrils composed of transthyretin and genetic typing established heterozygozity for the valine to isoleucine mutation at position 122 (Val122Ile). The diagnosis was therefore hereditary systemic amyloidosis as a result of a genetic transthyretin variant (ATTR) causing cardiac amyloidosis and coexistent asymptomatic myeloma. This requires symptomatic treatment of heart failure only. This article discusses a rare cause of heart failure and uses this case to illustrate that histological confirmation of the amyloid-causing protein is essential. Mistaken assumption of AL amyloid could have resulted in inappropriate cytotoxic therapy targeting the plasma cell clone.