diff --git a/cpu/cpu.cpp b/cpu/cpu.cpp
index dd84094..8196b53 100644
--- a/cpu/cpu.cpp
+++ b/cpu/cpu.cpp
@@ -258,7 +258,7 @@ void Cpu::add16(u16& out, u16 lhs, u16 rhs)
   u32 res32 = lhs + rhs;
   state.carry = (res32 & 0x10000);
   state.subtract = false;
-  lhs = (u16)res32;
+  out = (u16)res32;
 }
 
 bool Cpu::handleInterrupts()