diff --git a/structure_assignment/pipeline.py b/structure_assignment/pipeline.py
index a076ea0..9110bc5 100644
--- a/structure_assignment/pipeline.py
+++ b/structure_assignment/pipeline.py
@@ -87,7 +87,7 @@ class Runner:
                 tagged_columns = tagged_line.split('\t')
                 assert len(classla_columns) == 10, 'Missing token in classla-generated conllu ({}).'.format(len(tagged_line))
                 assert len(tagged_columns) == 10, 'Missing token in pre-tagged conllu ({}).'.format(len(classla_line))
-                assert classla_columns[1] == tagged_columns[1], 'Pre-tagged token form ({}) does not match classla-generated token form ({}).'.format(classla_tokens[0], tagged_columns[0])
+                assert classla_columns[1] == tagged_columns[1], 'Pre-tagged token form ({}) does not match classla-generated token form ({}).'.format(classla_columns[1], tagged_columns[1])
                 merged_columns = [classla_columns[i] if i in (3,5,9) else tagged_columns[i] for i in range(10)]
                 merged_line = '\t'.join(merged_columns)
             merged_conllu_file.write(merged_line + '\n')