diff --git a/bochs/cpu/proc_ctrl.cc b/bochs/cpu/proc_ctrl.cc
index 6fcee6dbe..b14a46439 100644
--- a/bochs/cpu/proc_ctrl.cc
+++ b/bochs/cpu/proc_ctrl.cc
@@ -499,7 +499,7 @@ bx_bool BX_CPU_C::is_monitor(bx_phy_address begin_addr, unsigned len)
   if (! BX_CPU_THIS_PTR monitor.armed) return 0;
 
   bx_phy_address monitor_begin = BX_CPU_THIS_PTR monitor.monitor_addr;
-  bx_phy_address monitor_end = monitor_start + CACHE_LINE_SIZE - 1;
+  bx_phy_address monitor_end = monitor_begin + CACHE_LINE_SIZE - 1;
 
   bx_phy_address end_addr = begin_addr + len;
   if (begin_addr >= monitor_end || end_addr <= monitor_begin)