diff --git a/packages/puter-js/src/modules/FileSystem/operations/upload.js b/packages/puter-js/src/modules/FileSystem/operations/upload.js
index b32da230..ab8778ea 100644
--- a/packages/puter-js/src/modules/FileSystem/operations/upload.js
+++ b/packages/puter-js/src/modules/FileSystem/operations/upload.js
@@ -122,7 +122,11 @@ const upload = async function(items, dirPath, options = {}){
                 entries[i].filepath = entries[i].name;
                 entries[i].fullPath = entries[i].name;
             }
-        }       
+        }
+        // Anything else is invalid
+        else {
+            return error({ code: 'field_invalid', message: 'upload() items parameter is an invalid type' });
+        }
 
         // Will hold directories and files to be uploaded
         let dirs = [];
diff --git a/packages/puter-js/src/modules/FileSystem/operations/write.js b/packages/puter-js/src/modules/FileSystem/operations/write.js
index 6ed2fd1a..c1319ad4 100644
--- a/packages/puter-js/src/modules/FileSystem/operations/write.js
+++ b/packages/puter-js/src/modules/FileSystem/operations/write.js
@@ -46,6 +46,11 @@ const write = async function (targetPath, data, options = {}) {
     if(!data)
         data = new File([data ?? ''], filename);
 
+    // data should be a File now. If it's not, it's an unsupported type
+    if (!(data instanceof File)) {
+        throw new Error({ code: 'field_invalid', message: 'write() data parameter is an invalid type' });
+    }
+
     // perform upload
     return this.upload(data, parent, options);
 }