diff --git a/dlib/svm/structural_graph_labeling_trainer.h b/dlib/svm/structural_graph_labeling_trainer.h
index 0259d568c..8f6cbc934 100644
--- a/dlib/svm/structural_graph_labeling_trainer.h
+++ b/dlib/svm/structural_graph_labeling_trainer.h
@@ -132,8 +132,14 @@ namespace dlib
             const std::vector<label_type>& labels
         ) const
         {
-            // TODO
             // make sure requires clause is not broken
+            DLIB_ASSERT(is_graph_labeling_problem(samples, labels),
+                "\t void structural_graph_labeling_trainer::train()"
+                << "\n\t samples.size(): " << samples.size() 
+                << "\n\t labels.size():  " << labels.size() 
+                << "\n\t this: " << this
+                );
+
 
             structural_svm_graph_labeling_problem<graph_type> prob(samples, labels, num_threads);