diff --git a/src/battle.c b/src/battle.c
index a63caa291..52d0456ad 100644
--- a/src/battle.c
+++ b/src/battle.c
@@ -1141,7 +1141,7 @@ terminate(troop dt, troop at, int type, const char *damage, bool missile)
     const weapon_type *dwtype = NULL;
     const weapon_type *awtype = NULL;
     const weapon *weapon;
-    variant res = frac_make(1, 1);
+    variant res = frac_one;
 
     int rda, sk = 0, sd;
     bool magic = false;
@@ -1185,6 +1185,7 @@ terminate(troop dt, troop at, int type, const char *damage, bool missile)
     }
 
     if (magic) {
+        res = frac_sub(frac_one, res);
         res = frac_mul(frac_make(da, 1), res);
         da = res.sa[0] / res.sa[1];
     }
diff --git a/src/items/weapons.c b/src/items/weapons.c
index acb47f0e0..53721bca6 100644
--- a/src/items/weapons.c
+++ b/src/items/weapons.c
@@ -76,7 +76,7 @@ int *casualties)
             killed += terminate(dt, *at, AT_SPELL, damage, 1);
         }
     } while (force && killed < enemies);
-    if (casualties)
+    if (killed > 0 && casualties)
         *casualties = killed;
     return true;
 }
diff --git a/src/kernel/build.c b/src/kernel/build.c
index 1f4e143df..81a6a90c8 100644
--- a/src/kernel/build.c
+++ b/src/kernel/build.c
@@ -405,7 +405,7 @@ static int required(int size, int msize, int maxneed)
 static int matmod(const unit * u, const resource_type * rtype, int value)
 {
     if (rtype->modifiers) {
-        variant save = frac_make(1, 1);
+        variant save = frac_one;
         const struct building_type *btype = NULL;
         const struct race *rc = u_race(u);
         resource_mod *mod;
diff --git a/src/volcano.c b/src/volcano.c
index db7166de0..c912bc0df 100644
--- a/src/volcano.c
+++ b/src/volcano.c
@@ -106,8 +106,8 @@ damage_unit(unit * u, const char *dam, bool physical, bool magic)
         int damage = dice_rand(dam);
         if (magic) {
             variant magres = magic_resistance(u);
-            magres = frac_sub(frac_make(1, 1), magres);
-            damage = damage * magres.sa[0] / magres.sa[1];
+            int save = magres.sa[0] / magres.sa[1];
+            damage -= damage * save;
         }
         if (physical) {
             damage -= nb_armor(u, i);