diff --git a/src/pkg/math/big/arith_amd64.s b/src/pkg/math/big/arith_amd64.s
index 088f724704..54f647322b 100644
--- a/src/pkg/math/big/arith_amd64.s
+++ b/src/pkg/math/big/arith_amd64.s
@@ -266,9 +266,9 @@ E7:	SUBL $1, BX		// i--
 TEXT ·bitLen(SB),7,$0
 	BSRQ x+0(FP), AX
 	JZ Z1
-	INCQ AX
-	MOVQ AX, n+8(FP)
+	INCL AX
+	MOVL AX, n+8(FP)
 	RET
 
-Z1:	MOVQ $0, n+8(FP)
+Z1:	MOVL $0, n+8(FP)
 	RET