diff --git a/handler.go b/handler.go
index cdda21e..a555f43 100644
--- a/handler.go
+++ b/handler.go
@@ -97,11 +97,6 @@ func (h *httpServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		h.errorHandler(ctx, nil, w, r, fmt.Errorf("path must starts with /"), http.StatusBadRequest)
 		return
 	}
-	cf, err := h.newCodec(ct)
-	if err != nil {
-		h.errorHandler(ctx, nil, w, r, err, http.StatusBadRequest)
-		return
-	}
 
 	matches := make(map[string]interface{})
 
@@ -166,6 +161,12 @@ func (h *httpServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	cf, err := h.newCodec(ct)
+	if err != nil {
+		h.errorHandler(ctx, nil, w, r, err, http.StatusBadRequest)
+		return
+	}
+
 	var argv, replyv reflect.Value
 
 	// Decode the argument value.