diff --git a/packages/@headlessui-react/CHANGELOG.md b/packages/@headlessui-react/CHANGELOG.md
index f2a1259258..349f117a50 100644
--- a/packages/@headlessui-react/CHANGELOG.md
+++ b/packages/@headlessui-react/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Ensure `Element` is available before polyfilling to prevent crashes in non-browser environments ([#3493](https://github.com/tailwindlabs/headlessui/pull/3493))
+- Fix crash when using `instanceof HTMLElement` in some environments ([#3494](https://github.com/tailwindlabs/headlessui/pull/3494))
 
 ## [2.1.8] - 2024-09-12
 
diff --git a/packages/@headlessui-react/src/internal/floating.tsx b/packages/@headlessui-react/src/internal/floating.tsx
index 216e1f9f87..cc611d6bf6 100644
--- a/packages/@headlessui-react/src/internal/floating.tsx
+++ b/packages/@headlessui-react/src/internal/floating.tsx
@@ -133,17 +133,14 @@ export function useFloatingPanel(
   let stablePlacement = useMemo(
     () => placement,
     [
-      JSON.stringify(
-        placement,
-        typeof HTMLElement !== 'undefined'
-          ? (_, v) => {
-              if (v instanceof HTMLElement) {
-                return v.outerHTML
-              }
-              return v
-            }
-          : undefined
-      ),
+      JSON.stringify(placement, (_, v) => {
+        // When we are trying to stringify a DOM element, we want to return the
+        // `outerHTML` of the element. In all other cases, we want to return the
+        // value as-is.
+        // It's not safe enough to check whether `v` is an instanceof
+        // `HTMLElement` because some tools (like AG Grid) polyfill it to be `{}`.
+        return v?.outerHTML ?? v
+      }),
     ]
   )
   useIsoMorphicEffect(() => {