--- a/inst/helintrlv.m
+++ b/inst/helintrlv.m
@@ -71,3 +71,10 @@
   endif
 
 endfunction
+
+%% Test input validation
+%!error helintrlv ()
+%!error helintrlv (1)
+%!error helintrlv (1, 2)
+%!error helintrlv (1, 2, 3)
+%!error helintrlv (1, 2, 3, 4, 5, 6)