diff --git a/pytorch_lightning/metrics/functional/classification.py b/pytorch_lightning/metrics/functional/classification.py
index e551046a38a43..096569a81f493 100644
--- a/pytorch_lightning/metrics/functional/classification.py
+++ b/pytorch_lightning/metrics/functional/classification.py
@@ -171,12 +171,12 @@ def stat_scores_multiple_classes(
         >>> sups
         tensor([1., 0., 1., 1.])
     """
-    num_classes = get_num_classes(pred=pred, target=target,
-                                  num_classes=num_classes)
-
     if pred.ndim == target.ndim + 1:
         pred = to_categorical(pred, argmax_dim=argmax_dim)
 
+    num_classes = get_num_classes(pred=pred, target=target,
+                                  num_classes=num_classes)
+
     tps = torch.zeros((num_classes,), device=pred.device)
     fps = torch.zeros((num_classes,), device=pred.device)
     tns = torch.zeros((num_classes,), device=pred.device)