diff --git a/include/openmc/material.h b/include/openmc/material.h
index 652f3db8e1..70c6b38522 100644
--- a/include/openmc/material.h
+++ b/include/openmc/material.h
@@ -132,7 +132,7 @@ public:
 
   //----------------------------------------------------------------------------
   // Data
-  int32_t id_; //!< Unique ID
+  int32_t id_ {-1}; //!< Unique ID
   std::string name_; //!< Name of material
   std::vector<int> nuclide_; //!< Indices in nuclides vector
   std::vector<int> element_; //!< Indices in elements vector
diff --git a/src/material.cpp b/src/material.cpp
index 685fd1da2e..9a1e206ac8 100644
--- a/src/material.cpp
+++ b/src/material.cpp
@@ -873,8 +873,8 @@ void Material::set_id(int32_t id)
   // If no ID specified, auto-assign next ID in sequence
   if (id == -1) {
     id = 0;
-    for (const auto& f : model::materials) {
-      id = std::max(id, f->id_);
+    for (const auto& m : model::materials) {
+      id = std::max(id, m->id_);
     }
     ++id;
   }