diff --git a/src/rational.h b/src/rational.h
index bab02e12130ae0e51730b2c2387a30c5efa3f55a..4544c23add947d8ca1b9af36e1ccedbd7de00a51 100644
--- a/src/rational.h
+++ b/src/rational.h
@@ -45,7 +45,7 @@ public:
 
     // Define conversions to and from AVRational (equivalent)
     rational(AVRational r) : num_(r.num), den_(r.den) {};
-    operator AVRational() { return AVRational{num_, den_}; }
+    operator AVRational() const { return AVRational{num_, den_}; }
 
     // Normal copy constructors and assignment operators