diff --git a/liberty/LibertyBuilder.cc b/liberty/LibertyBuilder.cc
index c80a1de7..a68e559f 100644
--- a/liberty/LibertyBuilder.cc
+++ b/liberty/LibertyBuilder.cc
@@ -687,7 +687,7 @@ LibertyBuilder::makeTimingArc(TimingArcSet *set,
 			      RiseFall *to_rf,
 			      TimingModel *model)
 {
-  return new TimingArc(set, from_rf->asTransition(),
+  return new TimingArc(set, from_rf ? from_rf->asTransition() : nullptr,
 		       to_rf->asTransition(), model);
 }