diff --git a/toy-impl/README.md b/toy-impl/README.md
index 20931dc..5f10826 100644
--- a/toy-impl/README.md
+++ b/toy-impl/README.md
@@ -84,6 +84,9 @@ Generics are supported via type variables.
 >    different. Please don't do that right now, would be fixed in later more
 >    serious implementations.
 
+We require the function to be well typed under every possible substitution of
+the type variables.
+
 For generic type variables, you can't really do much with them, other than
 passing them around in parameters, dealing with their list, etc.