Only flatten items if they're actually arrays (just in case they aren't)

dkist/wcs/models.py

@@ -777,7 +777,7 @@ def evaluate(self, *inputs_):
         else:
             has_units = False
             input_values = inputs_
-        input_values = [item.flatten() for item in input_values]
+        input_values = [item.flatten() if isinstance(item, np.ndarray) else item for item in input_values]
         # round the index values, but clip them if they exceed the array bounds
         # the bounds are one less than the shape dimension value
         array_bounds = np.array(self.array_shape) - 1