diff --git a/src/runtime/atomic_arm.go b/src/runtime/atomic_arm.go
index 00cc183..75206ab 100644
--- a/src/runtime/atomic_arm.go
+++ b/src/runtime/atomic_arm.go
@@ -159,8 +159,8 @@
 	// Align down to 4 bytes and use 32-bit CAS.
 	uaddr := uintptr(unsafe.Pointer(addr))
 	addr32 := (*uint32)(unsafe.Pointer(uaddr &^ 3))
-	mask := 0xFF << ((uaddr & 3) * 8)      // little endian
-	word := uint32(v) << ((uaddr & 3) * 8) // little endian
+	word := uint32(v) << ((uaddr & 3) * 8)    // little endian
+	mask := uint32(0xFF) << ((uaddr & 3) * 8) // little endian
 	word |= ^mask
 	for {
 		old := *addr32