diff --git a/unsupported/Eigen/src/Eigenvalues/ArpackSelfAdjointEigenSolver.h b/unsupported/Eigen/src/Eigenvalues/ArpackSelfAdjointEigenSolver.h
index 54db9bf29b1692e32e09b6c4c8d5eee618645272..7d5faf6add4690a1d4cfc394a4f459a79d38375c 100644
--- a/unsupported/Eigen/src/Eigenvalues/ArpackSelfAdjointEigenSolver.h
+++ b/unsupported/Eigen/src/Eigenvalues/ArpackSelfAdjointEigenSolver.h
@@ -10,6 +10,8 @@
 #ifndef EIGEN_ARPACKGENERALIZEDSELFADJOINTEIGENSOLVER_H
 #define EIGEN_ARPACKGENERALIZEDSELFADJOINTEIGENSOLVER_H
 
+#include <iostream>
+
 #include "../../../../Eigen/Dense"
 
 // IWYU pragma: private