diff --git a/moses/src/Scope3Parser/VarSpanTrieBuilder.h b/moses/src/Scope3Parser/VarSpanTrieBuilder.h
index fbc8124d5..76063af34 100644
--- a/moses/src/Scope3Parser/VarSpanTrieBuilder.h
+++ b/moses/src/Scope3Parser/VarSpanTrieBuilder.h
@@ -39,7 +39,7 @@ class VarSpanTrieBuilder
   typedef std::vector<IntermediateVarSpanNode> NodeVec;
   struct NodeVecState
   {
-    size_t m_size;
+    std::size_t m_size;
     IntermediateVarSpanNode m_lastNode;
   };
   void Build(ApplicableRuleTrie &, NodeVec &, VarSpanNode &);