diff --git a/include/hack.h b/include/hack.h
index b1d26d0f2..13de5e507 100644
--- a/include/hack.h
+++ b/include/hack.h
@@ -673,17 +673,17 @@ enum getobj_callback_returns {
 #define BZ_OFS_WAN(otyp) (abs((otyp) - WAN_MAGIC_MISSILE) % 10)
 #define BZ_OFS_SPE(otyp) (abs((otyp) - SPE_MAGIC_MISSILE) % 10)
 /* hero shooting a wand */
-#define BZ_U_WAND(bztyp) (0 + (bztyp))
+#define BZ_U_WAND(bztyp) (0 + (bztyp))     /*  0..9  */
 /* hero casting a spell */
-#define BZ_U_SPELL(bztyp) (10 + (bztyp))
+#define BZ_U_SPELL(bztyp) (10 + (bztyp))   /* 10..19 */
 /* hero breathing as a monster */
-#define BZ_U_BREATH(bztyp) (20 + (bztyp))
-/* monster shooting a wand */
-#define BZ_M_WAND(bztyp) (-0 - (bztyp))
+#define BZ_U_BREATH(bztyp) (20 + (bztyp))  /* 20..29 */
 /* monster casting a spell */
-#define BZ_M_SPELL(bztyp) (-10 - (bztyp))
+#define BZ_M_SPELL(bztyp) (-10 - (bztyp))  /* -19..-10 */
 /* monster breathing */
-#define BZ_M_BREATH(bztyp) (-20 - (bztyp))
+#define BZ_M_BREATH(bztyp) (-20 - (bztyp)) /* -29..-20 */
+/* monster shooting a wand; note: not -9 to -0 because -0 is ambiguous  */
+#define BZ_M_WAND(bztyp) (-30 - (bztyp))   /* -39..-30 */
 
 /*
  * option setting restrictions
diff --git a/src/zap.c b/src/zap.c
index 0658c83e9..c6fc893bc 100644
--- a/src/zap.c
+++ b/src/zap.c
@@ -55,12 +55,10 @@ static void wishcmdassist(int);
 static const char are_blinded_by_the_flash[] = "are blinded by the flash!";
 
 /*
- * FIXME:
- *  flash_types[0] for wand of magic missile is ambiguous.
- *  A positive index means zapped/cast/breathed by hero.
- *  A negative index means zapped/cast/breathed by a monster.
- *  Since abs(-0)==abs(0), there's no way to tell who zapped a wand of
- *  magic missile by just checking the index.
+ * A positive index means zapped/cast/breathed by hero.
+ * A negative index means zapped/cast/breathed by a monster, with value
+ * index fixup beyond abs() needed for wand zaps.  Wand zaps for monster
+ * use -39..-30 rather than -9..-0 because -0 is ambiguous (same as 0).
  */
 static const char *const flash_types[] = {
     "magic missile", /* Wands must be 0-9 */