diff --git a/supervision/detection/annotate.py b/supervision/detection/annotate.py
index de6e314f..f8bfff55 100644
--- a/supervision/detection/annotate.py
+++ b/supervision/detection/annotate.py
@@ -57,7 +57,7 @@ class BoxAnnotator:
         font = cv2.FONT_HERSHEY_SIMPLEX
         for i, (xyxy, confidence, class_id, tracker_id) in enumerate(detections):
             x1, y1, x2, y2 = xyxy.astype(int)
-            idx = class_id if not None else i
+            idx = class_id if class_id is not None else i
             color = (
                 self.color.by_idx(idx)
                 if isinstance(self.color, ColorPalette)