diff --git a/doc/fixes36.0 b/doc/fixes36.0
index b998d2c35..3c1e14a1b 100644
--- a/doc/fixes36.0
+++ b/doc/fixes36.0
@@ -924,6 +924,8 @@ you shouldn't see Sting glow light blue if you're blind
 when jumping, bumping into something is noisy
 flesh golems hit by electricity healed by wrong amount
 fleeing monsters couldn't use stairs that lead to different dungeon branch
+casting spell of protection when previous casting(s) hadn't time out yet
+	miscalculated the new AC increment
 
 
 Platform- and/or Interface-Specific Fixes
diff --git a/src/spell.c b/src/spell.c
index d8c7a1b32..877fb255a 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -1,4 +1,4 @@
-/* NetHack 3.6	spell.c	$NHDT-Date: 1434421353 2015/06/16 02:22:33 $  $NHDT-Branch: master $:$NHDT-Revision: 1.63 $ */
+/* NetHack 3.6	spell.c	$NHDT-Date: 1444295991 2015/10/08 09:19:51 $  $NHDT-Branch: master $:$NHDT-Revision: 1.64 $ */
 /*	Copyright (c) M. Stephenson 1988			  */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -720,10 +720,10 @@ int booktype;
 STATIC_OVL void
 cast_protection()
 {
-    int loglev = 0;
-    int l = u.ulevel;
-    int natac = u.uac - u.uspellprot;
-    int gain;
+    int l = u.ulevel, loglev = 0,
+        gain, natac = u.uac + u.uspellprot;
+    /* note: u.uspellprot is subtracted when find_ac() factors it into u.uac,
+       so adding here factors it back out (3.4.3,3.5 had this backwards) */
 
     /* loglev=log2(u.ulevel)+1 (1..5) */
     while (l) {
@@ -752,7 +752,8 @@ cast_protection()
      *     16-30   0    0,  5,  9, 11, 13, 14, 15
      *     16-30 -10    0,  5,  8,  9, 10
      */
-    gain = loglev - (int) u.uspellprot / (4 - min(3, (10 - natac) / 10));
+    natac = (10 - natac) / 10; /* convert to positive and scale down */
+    gain = loglev - (int) u.uspellprot / (4 - min(3, natac));
 
     if (gain > 0) {
         if (!Blind) {
@@ -763,28 +764,30 @@ cast_protection()
                 pline_The("%s haze around you becomes more dense.", hgolden);
             } else {
                 rmtyp = levl[u.ux][u.uy].typ;
-                atmosphere =
-                    u.uswallow
-                        ? ((u.ustuck->data == &mons[PM_FOG_CLOUD])
-                               ? "mist"
-                               : is_whirly(u.ustuck->data)
-                                     ? "maelstrom"
-                                     : is_animal(u.ustuck->data) ? "maw"
-                                                                 : "ooze")
-                        : u.uinwater ? "water" : (rmtyp == CLOUD)
-                                                     ? "cloud"
-                                                     : IS_TREE(rmtyp)
-                                                           ? "vegitation"
-                                                           : IS_STWALL(rmtyp)
-                                                                 ? "stone"
-                                                                 : "air";
+                atmosphere = u.uswallow
+                                ? ((u.ustuck->data == &mons[PM_FOG_CLOUD])
+                                   ? "mist"
+                                   : is_whirly(u.ustuck->data)
+                                      ? "maelstrom"
+                                      : is_animal(u.ustuck->data)
+                                         ? "maw"
+                                         : "ooze")
+                                : (u.uinwater
+                                   ? "water"
+                                   : (rmtyp == CLOUD)
+                                      ? "cloud"
+                                      : IS_TREE(rmtyp)
+                                         ? "vegitation"
+                                         : IS_STWALL(rmtyp)
+                                            ? "stone"
+                                            : "air");
                 pline_The("%s around you begins to shimmer with %s haze.",
                           atmosphere, an(hgolden));
             }
         }
         u.uspellprot += gain;
-        u.uspmtime =
-            P_SKILL(spell_skilltype(SPE_PROTECTION)) == P_EXPERT ? 20 : 10;
+        u.uspmtime = (P_SKILL(spell_skilltype(SPE_PROTECTION)) == P_EXPERT)
+                        ? 20 : 10;
         if (!u.usptime)
             u.usptime = u.uspmtime;
         find_ac();