diff --git a/AutoDiffHelpers.hpp b/AutoDiffHelpers.hpp
index 3e6c063c3..928fe64d4 100644
--- a/AutoDiffHelpers.hpp
+++ b/AutoDiffHelpers.hpp
@@ -248,19 +248,21 @@ subset(const Eigen::Array<Scalar, Eigen::Dynamic, 1>& x,
 }
 
 
-AutoDiff::ForwardBlock<double>::M
-spdiag(const AutoDiff::ForwardBlock<double>::V& d)
-{
-    typedef AutoDiff::ForwardBlock<double>::M M;
+namespace {
+    AutoDiff::ForwardBlock<double>::M
+    spdiag(const AutoDiff::ForwardBlock<double>::V& d)
+    {
+        typedef AutoDiff::ForwardBlock<double>::M M;
 
-    const int n = d.size();
-    M mat(n, n);
-    mat.reserve(Eigen::ArrayXi::Ones(n, 1));
-    for (M::Index i = 0; i < n; ++i) {
-        mat.insert(i, i) = d[i];
+        const int n = d.size();
+        M mat(n, n);
+        mat.reserve(Eigen::ArrayXi::Ones(n, 1));
+        for (M::Index i = 0; i < n; ++i) {
+            mat.insert(i, i) = d[i];
+        }
+
+        return mat;
     }
-
-    return mat;
-}
+} // Anonymous namespace
 
 #endif // OPM_AUTODIFFHELPERS_HEADER_INCLUDED