diff --git a/power/Power.cc b/power/Power.cc
index 5c1ebbc0..dbf55a27 100644
--- a/power/Power.cc
+++ b/power/Power.cc
@@ -490,9 +490,11 @@ PropActivityVisitor::visit(Vertex *vertex)
     }
     if (network_->isDriver(pin)) {
       LibertyPort *port = network_->libertyPort(pin);
-      LibertyCell *test_cell = port->libertyCell()->testCell();
-      if (test_cell)
-	port = test_cell->findLibertyPort(port->name());
+      if (port) {
+        LibertyCell *test_cell = port->libertyCell()->testCell();
+        if (test_cell)
+          port = test_cell->findLibertyPort(port->name());
+      }
       if (port) {
 	FuncExpr *func = port->function();
 	if (func) {