diff --git a/ultralytics/utils/metrics.py b/ultralytics/utils/metrics.py
index b5988110..ae98fb71 100644
--- a/ultralytics/utils/metrics.py
+++ b/ultralytics/utils/metrics.py
@@ -581,6 +581,7 @@ def ap_per_class(
         n_l = nt[ci]  # number of labels
         n_p = i.sum()  # number of predictions
         if n_p == 0 or n_l == 0:
+            prec_values.append(np.zeros(1000))
             continue
 
         # Accumulate FPs and TPs