diff --git a/common/timing_opt.cc b/common/timing_opt.cc
index a451bfa0..c0968db7 100644
--- a/common/timing_opt.cc
+++ b/common/timing_opt.cc
@@ -60,7 +60,7 @@ template <> struct hash<std::pair<int, NEXTPNR_NAMESPACE_PREFIX BelId>>
         return seed;
     }
 };
-
+#ifndef ARCH_GENERIC
 template <> struct hash<std::pair<NEXTPNR_NAMESPACE_PREFIX IdString, NEXTPNR_NAMESPACE_PREFIX BelId>>
 {
     std::size_t
@@ -72,6 +72,7 @@ template <> struct hash<std::pair<NEXTPNR_NAMESPACE_PREFIX IdString, NEXTPNR_NAM
         return seed;
     }
 };
+#endif
 } // namespace std
 
 NEXTPNR_NAMESPACE_BEGIN