diff --git a/js/src/jit/x86-shared/Lowering-x86-shared.cpp b/js/src/jit/x86-shared/Lowering-x86-shared.cpp
index aa4a8edf77f5..17988759c029 100644
--- a/js/src/jit/x86-shared/Lowering-x86-shared.cpp
+++ b/js/src/jit/x86-shared/Lowering-x86-shared.cpp
@@ -704,10 +704,11 @@ LIRGenerator::visitCopySign(MCopySign* ins)
 
     // As lowerForFPU, but we want rhs to be in a FP register too.
     lir->setOperand(0, useRegisterAtStart(lhs));
-    lir->setOperand(1, lhs != rhs ? useRegister(rhs) : useRegisterAtStart(rhs));
     if (!Assembler::HasAVX()) {
+        lir->setOperand(1, lhs != rhs ? useRegister(rhs) : useRegisterAtStart(rhs));
         defineReuseInput(lir, ins, 0);
     } else {
+        lir->setOperand(1, useRegisterAtStart(rhs));
         define(lir, ins);
     }
 }