diff --git a/goal_src/jak2/engine/collide/collide-shape-h.gc b/goal_src/jak2/engine/collide/collide-shape-h.gc
index 24cb16b97..1b30d5a20 100644
--- a/goal_src/jak2/engine/collide/collide-shape-h.gc
+++ b/goal_src/jak2/engine/collide/collide-shape-h.gc
@@ -671,3 +671,21 @@
         )
 
 (define-perm *collide-rider-pool* collide-rider-pool (new 'global 'collide-rider-pool))
+
+(defmacro normalized-heading-to-quaternion! (quat heading)
+  "Modified for PC. This does a clever trick, but it doesn't work if the heading is exactly
+   [0, 0, -1] because this tries to normalize a quaternion that's all 0's.
+   I think that, on ps2, something prevents the heading from being exactly 0, 0, -1,
+   but this doesn't happen on PC. A similar thing happened with Jak 1's punch glitch,
+   where some trig functions were slightly off from returning exactly 0.0 and 1.0 on ps2
+   rounding only."
+  `(begin
+     (if (and (= (-> ,heading x) 0.0)
+              (= (+ 1.0 (-> ,heading z)) 0.0)
+              )
+         (quaternion-set! ,quat 0.0 1.0 0.0 0.0)
+         (quaternion-set! ,quat 0.0 (-> ,heading x) 0.0 (+ 1.0 (-> ,heading z)))
+         )
+     (quaternion-normalize! ,quat)
+     )
+  )
\ No newline at end of file
diff --git a/goal_src/jak2/engine/nav/nav-enemy.gc b/goal_src/jak2/engine/nav/nav-enemy.gc
index a02bec662..51e35b354 100644
--- a/goal_src/jak2/engine/nav/nav-enemy.gc
+++ b/goal_src/jak2/engine/nav/nav-enemy.gc
@@ -559,21 +559,9 @@
       )
     (set! (-> s5-0 y) 0.0)
     (vector-normalize! s5-0 1.0)
-    (quaternion-set! (-> obj root-override2 quat) 0.0 (-> s5-0 x) 0.0 (+ 1.0 (-> s5-0 z)))
 
-    ;; modified for PC:
-    ;; this code is doing a clever way to go from heading vector to quaternion, but it doesn't handle the case where
-    ;; the heading is exactly pointing along -z. So we do this manually
-    ;; There's two possible theories for how this worked on ps2:
-    ;; - it was just wrong, but instead of giving nans, it just points the guard in the wrong direction
-    ;; - the code that generates the heading on PS2 can't generate (0, 0, -1) exactly.
-    ;;    (this was the case for punch glitch - the trig function acos couldn't reach 1.0 exactly with PS2
-    ;;     float rounding rules).
-    (when (and (= (-> s5-0 x) 0.0)
-               (= (+ 1.0 (-> s5-0 z)) 0.0) ;; actually the condition that causes problems
-               )
-      (quaternion-set! (-> obj root-override2 quat) 0.0 1.0 0.0 0.0)
-      )
+    ;; modified for PC, see comment near definition in collide-shape-h.gc
+    (normalized-heading-to-quaternion! (-> obj root-override2 quat) s5-0)
     )
   (quaternion-normalize! (-> obj root-override2 quat))
   0
diff --git a/goal_src/jak2/levels/city/common/citizen.gc b/goal_src/jak2/levels/city/common/citizen.gc
index 3f2643a87..513de4b71 100644
--- a/goal_src/jak2/levels/city/common/citizen.gc
+++ b/goal_src/jak2/levels/city/common/citizen.gc
@@ -610,8 +610,8 @@
       (let ((gp-0 (new 'stack-no-clear 'quaternion))
             (s5-1 (-> obj root-override2 quat))
             )
-        (quaternion-set! gp-0 0.0 (-> s3-0 x) 0.0 (+ 1.0 (-> s3-0 z)))
-        (quaternion-normalize! gp-0)
+        ;; modified for PC, see comment near definition in collide-shape-h.gc
+        (normalized-heading-to-quaternion! gp-0 s3-0)
         (quaternion-pseudo-seek
           s5-1
           s5-1
diff --git a/goal_src/jak2/levels/stadium/stadium-obs.gc b/goal_src/jak2/levels/stadium/stadium-obs.gc
index 41cfdbd49..93fc9388e 100644
--- a/goal_src/jak2/levels/stadium/stadium-obs.gc
+++ b/goal_src/jak2/levels/stadium/stadium-obs.gc
@@ -1777,8 +1777,7 @@ This commonly includes things such as:
           )
         (set! (-> s4-0 y) 0.0)
         (vector-normalize! s4-0 1.0)
-        (quaternion-set! s5-0 0.0 (-> s4-0 x) 0.0 (+ 1.0 (-> s4-0 z)))
-        (quaternion-normalize! s5-0)
+        (normalized-heading-to-quaternion! s5-0 s4-0) ;; changed to avoid NaN issue
         (quaternion-slerp!
           (-> self root-override2 quat)
           (-> self root-override2 quat)