diff --git a/src/luamm.hh b/src/luamm.hh
index a7fab725..be17bf43 100644
--- a/src/luamm.hh
+++ b/src/luamm.hh
@@ -70,7 +70,7 @@ namespace lua {
     };
 
     // we reserve one upvalue for the function pointer
-    int upvalueindex(int n)
+    inline int upvalueindex(int n)
     { return lua_upvalueindex(n+1); }
 
     /*