diff --git a/gtsam/nonlinear/NonlinearOptimizer.h b/gtsam/nonlinear/NonlinearOptimizer.h
index c977fcf35..edc62ae48 100644
--- a/gtsam/nonlinear/NonlinearOptimizer.h
+++ b/gtsam/nonlinear/NonlinearOptimizer.h
@@ -109,7 +109,7 @@ public:
   double error() const;
 
   /// return number of iterations
-  int iterations() const;
+  size_t iterations() const;
 
   /// return values
   const Values& values() const;
diff --git a/gtsam/nonlinear/internal/NonlinearOptimizerState.h b/gtsam/nonlinear/internal/NonlinearOptimizerState.h
index d356d20d5..0c65d91f9 100644
--- a/gtsam/nonlinear/internal/NonlinearOptimizerState.h
+++ b/gtsam/nonlinear/internal/NonlinearOptimizerState.h
@@ -40,15 +40,15 @@ struct NonlinearOptimizerState {
   const double error;
 
   /** The number of optimization iterations performed. */
-  const unsigned int iterations;
+  const size_t iterations;
 
   virtual ~NonlinearOptimizerState() {}
 
-  NonlinearOptimizerState(const Values& values, double error, unsigned int iterations = 0)
+  NonlinearOptimizerState(const Values& values, double error, size_t iterations = 0)
       : values(values), error(error), iterations(iterations) {}
 
   // Constructor version that takes ownership of values
-  NonlinearOptimizerState(Values&& values, double error, unsigned int iterations = 0)
+  NonlinearOptimizerState(Values&& values, double error, size_t iterations = 0)
       : values(std::move(values)), error(error), iterations(iterations) {}
 };