diff --git a/include/extern.h b/include/extern.h
index 5538d1a86..bf65ddc9c 100644
--- a/include/extern.h
+++ b/include/extern.h
@@ -155,7 +155,7 @@ extern boolean undiscovered_artifact(xint16);
 extern int disp_artifact_discoveries(winid);
 extern void dump_artifact_info(winid);
 extern boolean artifact_hit(struct monst *, struct monst *, struct obj *,
-                            int *, int);
+                            int *, int) NONNULLARG2;
 extern int doinvoke(void);
 extern boolean finesse_ahriman(struct obj *);
 extern int arti_speak(struct obj *);
diff --git a/src/artifact.c b/src/artifact.c
index a63f6a0fd..49a357913 100644
--- a/src/artifact.c
+++ b/src/artifact.c
@@ -1586,6 +1586,7 @@ artifact_hit(
                 if (youattack) {
                     healup(drain, 0, FALSE, FALSE);
                 } else {
+                    assert(magr != 0);
                     magr->mhp += drain;
                     if (magr->mhp > magr->mhpmax)
                         magr->mhp = magr->mhpmax;