diff --git a/txscript/script.go b/txscript/script.go
index 4a0d6686..4397f774 100644
--- a/txscript/script.go
+++ b/txscript/script.go
@@ -355,9 +355,9 @@ func calcSignatureHash(script []parsedOpcode, hashType SigHashType, tx *wire.Msg
 	// The final hash is the double sha256 of both the serialized modified
 	// transaction and the hash type (encoded as a 4-byte little-endian
 	// value) appended.
-	var wbuf bytes.Buffer
-	txCopy.Serialize(&wbuf)
-	binary.Write(&wbuf, binary.LittleEndian, hashType)
+	wbuf := bytes.NewBuffer(make([]byte, 0, txCopy.SerializeSize()+4))
+	txCopy.Serialize(wbuf)
+	binary.Write(wbuf, binary.LittleEndian, hashType)
 	return chainhash.DoubleHashB(wbuf.Bytes())
 }