diff --git a/cara/apps/calculator/__init__.py b/cara/apps/calculator/__init__.py
index 015e5569..3a63ad04 100644
--- a/cara/apps/calculator/__init__.py
+++ b/cara/apps/calculator/__init__.py
@@ -30,7 +30,7 @@ class ConcentrationModel(RequestHandler):
         except Exception as err:
             if DEBUG:
                 import traceback
-                traceback.print_last()
+                print(traceback.format_exc())
             response_json = {'code': 400, 'error': f'Your request was invalid {err}'}
             self.set_status(400)
             self.finish(json.dumps(response_json))
diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index efef71a9..5b202406 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -56,6 +56,8 @@ class FormData:
                              ('volume_type', VOLUME_TYPES),
                              ('windows_open', WINDOWS_OPEN)]
         for key, valid_set in validation_tuples:
+            if key not in form_data:
+                raise ValueError(f"Missing key {key}")
             if form_data[key] not in valid_set:
                 raise ValueError(f"{form_data[key]} is not a valid value for {key}")