diff --git a/dlib/array/array_sort_abstract.h b/dlib/array/array_sort_abstract.h
index ddacca7ca..36f47e543 100644
--- a/dlib/array/array_sort_abstract.h
+++ b/dlib/array/array_sort_abstract.h
@@ -16,7 +16,8 @@ namespace dlib
 
         /*!
             REQUIREMENTS ON ARRAY_BASE
-                must be an implementation of array/array_kernel_abstract.h 
+                - must be an implementation of array/array_kernel_abstract.h 
+                - array_base::type must be a type with that is comparable via operator<
 
             POINTERS AND REFERENCES
                 sort() may invalidate pointers and references to internal data.
diff --git a/dlib/queue/queue_sort_abstract.h b/dlib/queue/queue_sort_abstract.h
index 75a18cc36..ead9cc173 100644
--- a/dlib/queue/queue_sort_abstract.h
+++ b/dlib/queue/queue_sort_abstract.h
@@ -17,8 +17,8 @@ namespace dlib
 
         /*!
             REQUIREMENTS ON QUEUE_BASE
-                queue_base is instantiated with type T and
-                is an implementation of queue/queue_kernel_abstract.h
+                - is an implementation of queue/queue_kernel_abstract.h
+                - queue_base::type must be a type with that is comparable via operator<
 
             POINTERS AND REFERENCES TO INTERNAL DATA
                 sort() may invalidate pointers and references to internal data.