diff --git a/src/SimulationRunner.cc b/src/SimulationRunner.cc
index 07255523b8..5cd5db461d 100644
--- a/src/SimulationRunner.cc
+++ b/src/SimulationRunner.cc
@@ -495,7 +495,8 @@ void SimulationRunner::ProcessSystemQueue()
 
   this->systemMgr->ActivatePendingSystems();
 
-  auto threadCount = this->systemMgr->SystemsPostUpdate().size() + 1u;
+  unsigned int threadCount = static_cast<unsigned int>(
+    this->systemMgr->SystemsPostUpdate().size() + 1u);
 
   igndbg << "Creating PostUpdate worker threads: "
     << threadCount << std::endl;