diff --git a/gtsam_unstable/nonlinear/ConcurrentBatchSmoother.cpp b/gtsam_unstable/nonlinear/ConcurrentBatchSmoother.cpp
index 2761e3af6..8606538bf 100644
--- a/gtsam_unstable/nonlinear/ConcurrentBatchSmoother.cpp
+++ b/gtsam_unstable/nonlinear/ConcurrentBatchSmoother.cpp
@@ -350,7 +350,7 @@ ConcurrentBatchSmoother::Result ConcurrentBatchSmoother::optimize() {
       std::cout << "using lambda = " << lambda << std::endl;
 
     result.iterations++;
-  } while(result.iterations < parameters_.maxIterations &&
+  } while(result.iterations < (size_t)parameters_.maxIterations &&
       !checkConvergence(parameters_.relativeErrorTol, parameters_.absoluteErrorTol, parameters_.errorTol, previousError, result.error, NonlinearOptimizerParams::SILENT));
 
   return result;