diff --git a/include/extern.h b/include/extern.h
index 62feb5e48..05b5eaea3 100644
--- a/include/extern.h
+++ b/include/extern.h
@@ -546,7 +546,7 @@ extern const char *pmname(struct permonst *, int);
 #endif
 extern const char *mon_pmname(struct monst *);
 extern const char *obj_pmname(struct obj *);
-boolean mapxy_valid(coordxy x, coordxy y);
+extern boolean mapxy_valid(coordxy, coordxy);
 
 /* ### do_wear.c ### */