diff --git a/include/extern.h b/include/extern.h
index d3ef197b9..7814f7572 100644
--- a/include/extern.h
+++ b/include/extern.h
@@ -2481,7 +2481,7 @@ extern void shopper_financial_report(void);
 extern int inhishop(struct monst *);
 extern struct monst *shop_keeper(char);
 extern boolean tended_shop(struct mkroom *);
-struct bill_x *onbill(struct obj *, struct monst *, boolean);
+extern boolean onshopbill(struct obj *, struct monst *, boolean);
 extern boolean is_unpaid(struct obj *);
 extern void delete_contents(struct obj *);
 extern void obfree(struct obj *, struct obj *);
diff --git a/src/hack.c b/src/hack.c
index 2a6acc237..f5c690123 100644
--- a/src/hack.c
+++ b/src/hack.c
@@ -452,8 +452,9 @@ moverock(void)
             if (costly && !costly_spot(rx, ry)) {
                 addtobill(otmp, FALSE, FALSE, FALSE);
             } else if (!costly && costly_spot(rx, ry) && otmp->unpaid
-                       && (shkp = shop_keeper(*in_rooms(rx, ry, SHOPBASE)))
-                       && onbill(otmp, shkp, TRUE)) {
+                       && ((shkp = shop_keeper(*in_rooms(rx, ry, SHOPBASE)))
+                           != 0)
+                       && onshopbill(otmp, shkp, TRUE)) {
                 subfrombill(otmp, shkp);
             }
         } else {
diff --git a/src/shk.c b/src/shk.c
index 866e2aebc..b9c58c54d 100644
--- a/src/shk.c
+++ b/src/shk.c
@@ -29,6 +29,7 @@ static const char the_contents_of[] = "the contents of ";
 static void append_honorific(char *);
 static long addupbill(struct monst *);
 static void pacify_shk(struct monst *, boolean);
+static struct bill_x *onbill(struct obj *, struct monst *, boolean);
 static struct monst *next_shkp(struct monst *, boolean);
 static long shop_debt(struct eshk *);
 static char *shk_owns(char *, struct obj *);
@@ -82,7 +83,9 @@ static const char *cad(boolean);
                     obj->quan <= bp->bquan
  */
 
-static const char *const angrytexts[] = { "quite upset", "ticked off", "furious" };
+static const char *const angrytexts[] = {
+    "quite upset", "ticked off", "furious"
+};
 
 /*
  *  Transfer money from inventory to monster when paying
@@ -879,14 +882,23 @@ onbill(struct obj *obj, struct monst *shkp, boolean silent)
                 if (!obj->unpaid)
                     pline("onbill: paid obj on bill?");
                 return bp;
-            } else
+            } else {
                 bp++;
+            }
     }
     if (obj->unpaid && !silent)
         pline("onbill: unpaid obj not on bill?");
     return (struct bill_x *) 0;
 }
 
+/* used outside of shk.c when caller wants to know whether item is on bill
+   but doesn't need to know any details about the bill itself */
+boolean
+onshopbill(struct obj *obj, struct monst *shkp, boolean silent)
+{
+    return onbill(obj, shkp, silent) ? TRUE : FALSE;
+}
+
 /* check whether an object or any of its contents belongs to a shop */
 boolean
 is_unpaid(struct obj* obj)