diff --git a/salsa20/salsa20.go b/salsa20/salsa20.go
index 0ee6248..3ca6748 100644
--- a/salsa20/salsa20.go
+++ b/salsa20/salsa20.go
@@ -32,7 +32,7 @@ import (
 // be either 8 or 24 bytes long.
 func XORKeyStream(out, in []byte, nonce []byte, key *[32]byte) {
 	if len(out) < len(in) {
-		in = in[:len(out)]
+		panic("salsa20: output smaller than input")
 	}
 
 	var subNonce [16]byte