diff --git a/gtsam/discrete/SignatureParser.cpp b/gtsam/discrete/SignatureParser.cpp
index 2081e1ee1..8e0221f01 100644
--- a/gtsam/discrete/SignatureParser.cpp
+++ b/gtsam/discrete/SignatureParser.cpp
@@ -34,7 +34,7 @@ std::optional<Row> static ParseConditional(const std::string& token) {
       row.push_back(std::stod(s));
     }
     // if we ended with a '/' then return false
-    if (token.back() == '/') return false;
+    if (token.back() == '/') return std::nullopt;
   } catch (...) {
     return std::nullopt;
   }