diff --git a/src/lib/crypto.pl b/src/lib/crypto.pl
index 71c0420e..bec12d51 100644
--- a/src/lib/crypto.pl
+++ b/src/lib/crypto.pl
@@ -711,6 +711,8 @@ curve25519_scalar_mult(Scalar, Point, Result) :-
             must_be_bytes(ScalarBytes, curve25519_scalar_mult/3),
             length(ScalarBytes, 32)
         ),
+        must_be(chars, Point),
+        length(Point, 32),
         maplist(char_code, Point, PointBytes),
         '$curve25519_scalar_mult'(ScalarBytes, PointBytes, Result).