diff --git a/sim_serial.h b/sim_serial.h
index 757c2713..b66f8219 100644
--- a/sim_serial.h
+++ b/sim_serial.h
@@ -73,8 +73,8 @@
 
 #endif  /* OS variants */
 
-#ifndef SERHANDLE_DEFINED
-#define SERHANDLE_DEFINED 0
+#ifndef SIMH_SERHANDLE_DEFINED
+#define SIMH_SERHANDLE_DEFINED 0
 #if defined (_WIN32)                            /* Windows definitions */
 typedef void *SERHANDLE;
 #else                                           /* all other platforms */