diff --git a/src/x86.cpp b/src/x86.cpp
index 3e5ec4c17f..2af36d001f 100644
--- a/src/x86.cpp
+++ b/src/x86.cpp
@@ -1762,7 +1762,7 @@ compareCR(Context* c, unsigned size, Assembler::Constant* a,
 }
 
 void
-compareCM(Context* c, unsigned size, Assembler::Constant* a,
+compareCM(Context* c, unsigned size UNUSED, Assembler::Constant* a,
           Assembler::Memory* b)
 {
   assert(c, BytesPerWord == 8 or size == 4);