diff --git a/dlib/svm/kkmeans_abstract.h b/dlib/svm/kkmeans_abstract.h
index 74fdbf4f1..e676bbdbf 100644
--- a/dlib/svm/kkmeans_abstract.h
+++ b/dlib/svm/kkmeans_abstract.h
@@ -109,6 +109,8 @@ namespace dlib
         );
         /*!
             requires
+                - matrix_type and matrix_type2 must either be dlib::matrix objects or convertable to dlib::matrix
+                  via vector_to_matrix()
                 - matrix_type::type == sample_type  (i.e. matrix_type should contain sample_type objects)
                 - matrix_type2::type == sample_type (i.e. matrix_type2 should contain sample_type objects)
                 - initial_centers.nc() == 1         (i.e. must be a column vector)