diff --git a/slirp/slirp.h b/slirp/slirp.h
index d57fb12363..6f8a7f6022 100644
--- a/slirp/slirp.h
+++ b/slirp/slirp.h
@@ -120,13 +120,12 @@ typedef unsigned char u_int8_t;
 #include <sys/uio.h>
 #endif
 
-#ifndef _P
+#undef _P
 #ifndef NO_PROTOTYPES
 #  define   _P(x)   x
 #else
 #  define   _P(x)   ()
 #endif
-#endif
 
 #ifndef _WIN32
 #include <netinet/in.h>