diff --git a/modloader.js b/modloader.js
index 8f43001..74f1a82 100644
--- a/modloader.js
+++ b/modloader.js
@@ -63,7 +63,11 @@ globalThis.modapi_modloader = "(" + (() => {
 
     globalThis.addFileMod = async function addFileMod(mod, textContents) {
         const mods = await getMods();
-        mods.push(mod);
+        if (mods.includes(mod)) {
+            await removeMod(mods.indexOf(mod));
+        } else {
+            mods.push(mod);
+        }
         await saveMods(mods);
 
         const db = await getDatabase();
diff --git a/postinit.js b/postinit.js
index 14e9698..fde5764 100644
--- a/postinit.js
+++ b/postinit.js
@@ -265,7 +265,11 @@ globalThis.modapi_postinit = "(" + (() => {
                     "class": item || null,
                     "hasMeta": !!item,
                     "instanceOf": function (object) {
-                        return ModAPI.hooks._teavm.$rt_isInstance(object, item || null);
+                        try {
+                            return ModAPI.hooks._teavm.$rt_isInstance(object, item || null);
+                        } catch {
+                            return false;
+                        }
                     },
                     "compiledName": compiledName
                 }