diff --git a/stanza/models/constituency/parse_tree.py b/stanza/models/constituency/parse_tree.py
index 372b3066cc..9dbaf6dc39 100644
--- a/stanza/models/constituency/parse_tree.py
+++ b/stanza/models/constituency/parse_tree.py
@@ -308,6 +308,8 @@ def get_unique_constituent_labels(trees):
         """
         Walks over all of the trees and gets all of the unique constituent names from the trees
         """
+        if isinstance(trees, Tree):
+            trees = [trees]
         constituents = Tree.get_constituent_counts(trees)
         return sorted(set(constituents.keys()))
 
diff --git a/stanza/models/constituency/trainer.py b/stanza/models/constituency/trainer.py
index dbf3913be1..0202c562fe 100644
--- a/stanza/models/constituency/trainer.py
+++ b/stanza/models/constituency/trainer.py
@@ -466,7 +466,15 @@ def check_constituents(train_constituents, trees, treebank_name):
     constituents = parse_tree.Tree.get_unique_constituent_labels(trees)
     for con in constituents:
         if con not in train_constituents:
-            raise RuntimeError("Found constituent label {} in the {} set which don't exist in the train set".format(con, treebank_name))
+            first_error = None
+            num_errors = 0
+            for tree_idx, tree in enumerate(trees):
+                constituents = parse_tree.Tree.get_unique_constituent_labels(tree)
+                if con in constituents:
+                    num_errors += 1
+                    if first_error is None:
+                        first_error = tree_idx
+            raise RuntimeError("Found constituent label {} in the {} set which don't exist in the train set.  This constituent label occured in {} trees, with the first tree index at {} counting from 1\nThe error tree (which may have POS tags changed from the retagger and may be missing functional tags or empty nodes) is:\n{:P}".format(con, treebank_name, num_errors, (first_error+1), trees[first_error]))
 
 def check_root_labels(root_labels, other_trees, treebank_name):
     """