diff --git a/opm/core/linalg/LinearSolverUmfpack.cpp b/opm/core/linalg/LinearSolverUmfpack.cpp
index 065489966..aec76f40a 100644
--- a/opm/core/linalg/LinearSolverUmfpack.cpp
+++ b/opm/core/linalg/LinearSolverUmfpack.cpp
@@ -48,8 +48,8 @@ namespace Opm
                                double* solution) const
     {
         CSRMatrix A  = {
-            size,
-            nonzeros,
+            (size_t)size,
+            (size_t)nonzeros,
             const_cast<int*>(ia),
             const_cast<int*>(ja),
             const_cast<double*>(sa)