diff --git a/src/com/xilinx/rapidwright/rwroute/RouteNodeGraph.java b/src/com/xilinx/rapidwright/rwroute/RouteNodeGraph.java
index 54247bac0..89e01ad87 100644
--- a/src/com/xilinx/rapidwright/rwroute/RouteNodeGraph.java
+++ b/src/com/xilinx/rapidwright/rwroute/RouteNodeGraph.java
@@ -329,7 +329,7 @@ public void preserve(Net net, List<SitePinInst> pins) {
                 } else if (pinName.endsWith("_O")) {
                     otherPinName = lutLetter + "MUX";
                 } else {
-                    throw new RuntimeException(pinName);
+                    throw new RuntimeException("ERROR: Unsupported site pin " + pin);
                 }
 
                 Node otherNode = si.getSite().getConnectedNode(otherPinName);