diff --git a/src/emu/machine/ncr539x.c b/src/emu/machine/ncr539x.c
index 577578be1c7..c93dd441bb5 100644
--- a/src/emu/machine/ncr539x.c
+++ b/src/emu/machine/ncr539x.c
@@ -340,6 +340,8 @@ READ8_MEMBER( ncr539x_device::read )
 							{
 								fifo_fill_size = m_xfer_count;
 							}
+							assert(m_buffer_offset < m_buffer_size);
+							assert((m_buffer_offset + fifo_fill_size) < m_buffer_size);
 							memcpy(m_fifo, &m_buffer[m_buffer_offset], fifo_fill_size);
 							m_buffer_offset += fifo_fill_size;
 							m_buffer_remaining -= fifo_fill_size;