Lab 4: fix string error

lab4/twidder/server.py

@@ -104,7 +104,7 @@ def is_signup_form_valid(data):
         return False
 
     for field in data:
-        if not (len(data[field].strip())):
+        if not (len(str(data[field]).strip())):
             # string is empty
             return False