diff --git a/cpp/src/api/convert.cpp b/cpp/src/api/convert.cpp
index c5be6f3d..2c23cbf4 100644
--- a/cpp/src/api/convert.cpp
+++ b/cpp/src/api/convert.cpp
@@ -34,7 +34,10 @@ std::optional<loot::EdgeType> convert(loot::rust::EdgeType edgeType) {
     case loot::rust::EdgeType::None:
       return std::nullopt;
     default:
-      return std::nullopt;
+      throw std::logic_error(
+          "Unrecognised EdgeType: " +
+          std::to_string(
+              std::underlying_type_t<loot::rust::EdgeType>(edgeType)));
   }
 }
 }