diff --git a/MathLib/LinAlg/Eigen/EigenVector.h b/MathLib/LinAlg/Eigen/EigenVector.h
index ffe5221060d1df338cbbb5c57d4c57c02f19c1a1..570356ed1902bd4b78970ec620e7750617e02322 100644
--- a/MathLib/LinAlg/Eigen/EigenVector.h
+++ b/MathLib/LinAlg/Eigen/EigenVector.h
@@ -43,7 +43,7 @@ public:
     EigenVector(EigenVector const& src) = default;
 
     /// return a vector length
-    IndexType size() const { return _vec.size(); }
+    IndexType size() const { return static_cast<IndexType>(_vec.size()); }
 
     /// return a start index of the active data range
     IndexType getRangeBegin() const { return 0;}