diff --git a/src/math/big/decimal.go b/src/math/big/decimal.go
index 2c0c9daebc..2dfa032c77 100644
--- a/src/math/big/decimal.go
+++ b/src/math/big/decimal.go
@@ -125,11 +125,12 @@ func shr(x *decimal, s uint) {
 
 	// read a digit, write a digit
 	w := 0 // write index
+	mask := Word(1)<<s - 1
 	for r < len(x.mant) {
 		ch := Word(x.mant[r])
 		r++
 		d := n >> s
-		n -= d << s
+		n &= mask // n -= d << s
 		x.mant[w] = byte(d + '0')
 		w++
 		n = n*10 + ch - '0'
@@ -138,7 +139,7 @@ func shr(x *decimal, s uint) {
 	// write extra digits that still fit
 	for n > 0 && w < len(x.mant) {
 		d := n >> s
-		n -= d << s
+		n &= mask
 		x.mant[w] = byte(d + '0')
 		w++
 		n = n * 10
@@ -148,7 +149,7 @@ func shr(x *decimal, s uint) {
 	// append additional digits that didn't fit
 	for n > 0 {
 		d := n >> s
-		n -= d << s
+		n &= mask
 		x.mant = append(x.mant, byte(d+'0'))
 		n = n * 10
 	}