diff --git a/opm/simulators/linalg/bda/opencl/openclKernels.cpp b/opm/simulators/linalg/bda/opencl/openclKernels.cpp
index b43f70e66..6ac7791b8 100644
--- a/opm/simulators/linalg/bda/opencl/openclKernels.cpp
+++ b/opm/simulators/linalg/bda/opencl/openclKernels.cpp
@@ -41,7 +41,7 @@ int OpenclKernels::verbosity;
 cl::CommandQueue *OpenclKernels::queue;
 std::vector<double> OpenclKernels::tmp;
 bool OpenclKernels::initialized = false;
-size_t OpenclKernels::preferred_workgroup_size_multiple = 0;
+std::size_t OpenclKernels::preferred_workgroup_size_multiple = 0;
 
 std::unique_ptr<cl::KernelFunctor<cl::Buffer&, cl::Buffer&, cl::Buffer&, const unsigned int, cl::LocalSpaceArg> > OpenclKernels::dot_k;
 std::unique_ptr<cl::KernelFunctor<cl::Buffer&, cl::Buffer&, const unsigned int, cl::LocalSpaceArg> > OpenclKernels::norm_k;
diff --git a/opm/simulators/linalg/bda/opencl/openclKernels.hpp b/opm/simulators/linalg/bda/opencl/openclKernels.hpp
index e85b2ff66..578d66bbd 100644
--- a/opm/simulators/linalg/bda/opencl/openclKernels.hpp
+++ b/opm/simulators/linalg/bda/opencl/openclKernels.hpp
@@ -22,6 +22,7 @@
 
 #include <string>
 #include <memory>
+#include <cstddef>
 
 #include <opm/simulators/linalg/bda/opencl/opencl.hpp>
 
@@ -64,7 +65,7 @@ private:
     static cl::CommandQueue *queue;
     static std::vector<double> tmp;     // used as tmp CPU buffer for dot() and norm()
     static bool initialized;
-    static size_t preferred_workgroup_size_multiple; // stores CL_KERNEL_PREFERRED_WORK_GROUP_SIZE_MULTIPLE
+    static std::size_t preferred_workgroup_size_multiple; // stores CL_KERNEL_PREFERRED_WORK_GROUP_SIZE_MULTIPLE
 
     static std::unique_ptr<cl::KernelFunctor<cl::Buffer&, cl::Buffer&, cl::Buffer&, const unsigned int, cl::LocalSpaceArg> > dot_k;
     static std::unique_ptr<cl::KernelFunctor<cl::Buffer&, cl::Buffer&, const unsigned int, cl::LocalSpaceArg> > norm_k;