diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index 4dd7be70..bd2114d5 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -76,7 +76,12 @@ class FormData:
             if form_data[key] not in valid_set:
                 raise ValueError(f"{form_data[key]} is not a valid value for {key}")
 
-        # Don't let arbirtrary unescaped HTML through the net.
+        if (form_data['ventilation_type'] == 'natural' and
+            form_data['window_type'] == 'not-applicable'):
+            raise ValueError("window_type cannot be ''not-applicable'' if "
+                             "ventilation_type is ''natural''")
+
+        # Don't let arbitrary unescaped HTML through the net.
         for key, value in form_data.items():
             if isinstance(value, str):
                 form_data[key] = html.escape(value)
diff --git a/cara/models.py b/cara/models.py
index 66234a26..4478c1ce 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -260,6 +260,10 @@ class HingedWindow(WindowOpening):
     #: Window width (m).
     window_width: float = None
 
+    def __post_init__(self):
+        if self.window_width is None:
+            raise ValueError('window_width must be set')
+
     @property
     def discharge_coefficient(self) -> float:
         """
@@ -284,7 +288,7 @@ class HingedWindow(WindowOpening):
         else:
             M = 0.038
             cd_max = 0.548
-        window_angle = 2.*np.arcsin(self.opening_length/(2.*self.window_height))*180/np.pi
+        window_angle = 2.*np.rad2deg(np.arcsin(self.opening_length/(2.*self.window_height)))
         return cd_max*(1-np.exp(-M*window_angle))