diff --git a/poly1305/poly1305_amd64.s b/poly1305/poly1305_amd64.s
index 523f5b9..431c432 100644
--- a/poly1305/poly1305_amd64.s
+++ b/poly1305/poly1305_amd64.s
@@ -121,6 +121,7 @@ done:
 	MOVQ    R9, BX
 	SUBQ    $0xFFFFFFFFFFFFFFFB, AX
 	SBBQ    $0xFFFFFFFFFFFFFFFF, BX
+	SBBQ    $3, R10
 	CMOVQCS R8, AX
 	CMOVQCS R9, BX
 	ADDQ    16(SP), AX