An indirect memory expression contained a combination of registers that was invalid for the selected addressing mode. More than one register was evaluated as a base register by the assembler; the processor only supports a single base register within register indirect addressing mode.
For expressions that refer to 16-bit (USE16) memory locations, only BX and BP are valid base registers.
For expressions that refer to 32-bit (USE32) memory locations, only EAX, EBX, ECX, EDX, ESP, EBP, EDI, and ESI are valid base registers.