diff --git a/src/Solvers/ArbitraryDomain.cpp b/src/Solvers/ArbitraryDomain.cpp
index 6da0f901ee5badf570d2fab443add44b3d89bffb..499f872ebc4315e2529749885bfaed9de3ccc080 100644
--- a/src/Solvers/ArbitraryDomain.cpp
+++ b/src/Solvers/ArbitraryDomain.cpp
@@ -35,6 +35,7 @@
 #include <iostream>
 #include <tuple>
 #include "Utilities/OpalException.h"
+#include "Index/NDIndex.h"
 
 ArbitraryDomain::ArbitraryDomain( BoundaryGeometry * bgeom,
                                   IntVector_t nr,
diff --git a/src/Solvers/IrregularDomain.h b/src/Solvers/IrregularDomain.h
index 9b865477d26138e6aa31d67c96347a4d00d418f4..ba45351d6eba68a640c13b4dfe74dbebc823aa72 100644
--- a/src/Solvers/IrregularDomain.h
+++ b/src/Solvers/IrregularDomain.h
@@ -30,6 +30,7 @@
 #include <string>
 #include "Algorithms/Vektor.h"
 #include "Algorithms/Quaternion.h"
+#include "Index/NDIndex.h"
 
 /// enumeration corresponding to different interpolation methods at the boundary
 enum {