diff --git a/include/extern.h b/include/extern.h
index b9f2e5cce..e372f52ff 100644
--- a/include/extern.h
+++ b/include/extern.h
@@ -1922,8 +1922,8 @@ extern boolean accessible(coordxy, coordxy);
 extern void set_apparxy(struct monst *) NONNULLARG1;
 extern boolean can_ooze(struct monst *) NONNULLARG1;
 extern boolean can_fog(struct monst *) NONNULLARG1;
-extern boolean should_displace(struct monst *, struct mfndposdata *, coordxy,
-                               coordxy) NONNULLPTRS;
+extern boolean should_displace(struct monst *, const struct mfndposdata *,
+                               coordxy, coordxy) NONNULLPTRS;
 extern boolean undesirable_disp(struct monst *, coordxy, coordxy) NONNULLARG1;
 extern boolean can_hide_under_obj(struct obj *);
 
diff --git a/src/monmove.c b/src/monmove.c
index 2721b54a0..3830e3713 100644
--- a/src/monmove.c
+++ b/src/monmove.c
@@ -1069,7 +1069,7 @@ itsstuck(struct monst *mtmp)
 boolean
 should_displace(
     struct monst *mtmp,
-    struct mfndposdata *data,
+    const struct mfndposdata *data,
     coordxy ggx,
     coordxy ggy)
 {