diff --git a/cellpose/metrics.py b/cellpose/metrics.py
index a33aabf5..ad233eac 100644
--- a/cellpose/metrics.py
+++ b/cellpose/metrics.py
@@ -118,8 +118,8 @@ def average_precision(masks_true, masks_pred, threshold=[0.5, 0.75, 0.9]):
     tp = np.zeros((len(masks_true), len(threshold)), np.float32)
     fp = np.zeros((len(masks_true), len(threshold)), np.float32)
     fn = np.zeros((len(masks_true), len(threshold)), np.float32)
-    n_true = np.array(list(map(np.max, masks_true)))
-    n_pred = np.array(list(map(np.max, masks_pred)))
+    n_true = np.array([len(np.unique(mt)) - 1 for mt in masks_true])
+    n_pred = np.array([len(np.unique(mp)) - 1 for mp in masks_pred])
 
     for n in range(len(masks_true)):
         #_,mt = np.reshape(np.unique(masks_true[n], return_index=True), masks_pred[n].shape)