diff --git a/gtsam/inference/LabeledSymbol.cpp b/gtsam/inference/LabeledSymbol.cpp
index 6e7440402..c187e864e 100644
--- a/gtsam/inference/LabeledSymbol.cpp
+++ b/gtsam/inference/LabeledSymbol.cpp
@@ -72,7 +72,8 @@ void LabeledSymbol::print(const std::string& s) const {
 /* ************************************************************************* */
 LabeledSymbol::operator std::string() const {
   char buffer[100];
-  snprintf(buffer, 100, "%c%c%lu", c_, label_, j_);
+  snprintf(buffer, 100, "%c%c%llu", c_, label_,
+           static_cast<unsigned long long>(j_));
   return std::string(buffer);
 }
 
diff --git a/gtsam/inference/Symbol.cpp b/gtsam/inference/Symbol.cpp
index 000553d8c..24af9d9f6 100644
--- a/gtsam/inference/Symbol.cpp
+++ b/gtsam/inference/Symbol.cpp
@@ -57,7 +57,7 @@ bool Symbol::equals(const Symbol& expected, double tol) const {
 
 Symbol::operator std::string() const {
   char buffer[10];
-  snprintf(buffer, 10, "%c%lu", c_, j_);
+  snprintf(buffer, 10, "%c%llu", c_, static_cast<unsigned long long>(j_));
   return std::string(buffer);
 }