small fix

mot-metrics.py

@@ -222,7 +222,10 @@ def calculate_from_payload(payload: dict, max_iou: float = 0.5, debug: bool = Fa
             if debug:
                 print("formated_predictions: ", formated_predictions)
                 print("formatted_references: ", formatted_references)
-            output[sequence][model] = calculate(formated_predictions, formatted_references, max_iou=max_iou)
+            if len(formated_predictions) == 0:
+                output[sequence][model] = "Model had no predictions."
+            else:
+                output[sequence][model] = calculate(formated_predictions, formatted_references, max_iou=max_iou)
     return output