diff --git a/app/common/rational.cpp b/app/common/rational.cpp
index 8d12a69a1..fd5641047 100644
--- a/app/common/rational.cpp
+++ b/app/common/rational.cpp
@@ -142,7 +142,9 @@ rational rational::flipped() const
 
 void rational::flip()
 {
-  std::swap(denom_, numer_);
+  if (!isNull()) {
+    std::swap(denom_, numer_);
+  }
 }
 
 bool rational::isNull() const