diff --git a/src/xenia/kernel/xthread.cc b/src/xenia/kernel/xthread.cc
index 7e00efb4b..c269c1806 100644
--- a/src/xenia/kernel/xthread.cc
+++ b/src/xenia/kernel/xthread.cc
@@ -704,6 +704,7 @@ uint32_t XThread::active_cpu() const {
 }
 
 void XThread::SetActiveCpu(uint32_t cpu_index) {
+  assert_true(cpu_index < 6);
   uint8_t* pcr = memory()->TranslateVirtual(pcr_address_);
   xe::store_and_swap<uint8_t>(pcr + 0x10C, cpu_index);
 }