Generated incorrect .asm instruction

rc4.c line 34 "x = x + state[i] + pKey[mod_16x8(i, iKeyLen)];" can not compile, change to "x += state[i] + pKey[mod_16x8(i, iKeyLen)];" ok

C:\sdcc\bin>sdcc rc4.c -mz80 -c
rc4.asm:123: Error: <a> machine specific addressing or addressing mode error
removing rc4.rel

C:\sdcc\bin>sdcc -v
SDCC : mcs51/z80 3.0.1 #6066 (Nov 25 2010) (MINGW32)