diff --git a/lib/dynasm-backend/src/codegen_x64.rs b/lib/dynasm-backend/src/codegen_x64.rs
index 6742b64f0..5a1f122e5 100644
--- a/lib/dynasm-backend/src/codegen_x64.rs
+++ b/lib/dynasm-backend/src/codegen_x64.rs
@@ -99,7 +99,7 @@ impl Register {
             9 => R12,
             10 => R13,
             11 => R14,
-            12 => R15,
+            // 12 => R15, // R15 is reserved for memory base pointer.
             _ => unreachable!(),
         }
     }