diff --git a/include/bout/interpolation_xz.hxx b/include/bout/interpolation_xz.hxx
index 8a815dfb6b..d8dbeebc7d 100644
--- a/include/bout/interpolation_xz.hxx
+++ b/include/bout/interpolation_xz.hxx
@@ -273,4 +273,12 @@ using RegisterXZInterpolation = XZInterpolationFactory::RegisterInFactory<Derive
 using RegisterUnavailableXZInterpolation =
     XZInterpolationFactory::RegisterUnavailableInFactory;
 
+namespace {
+RegisterXZInterpolation<XZHermiteSpline> registerinterphermitespline{"hermitespline"};
+RegisterXZInterpolation<XZMonotonicHermiteSpline> registerinterpmonotonichermitespline{
+    "monotonichermitespline"};
+RegisterXZInterpolation<XZLagrange4pt> registerinterplagrange4pt{"lagrange4pt"};
+RegisterXZInterpolation<XZBilinear> registerinterpbilinear{"bilinear"};
+} // namespace
+
 #endif // INTERP_XZ_H
diff --git a/src/mesh/interpolation_xz.cxx b/src/mesh/interpolation_xz.cxx
index 7dc48eca90..d151c35613 100644
--- a/src/mesh/interpolation_xz.cxx
+++ b/src/mesh/interpolation_xz.cxx
@@ -85,11 +85,3 @@ const Field3D interpolate(const Field2D& f, const Field3D& delta_x) {
   }
   return result;
 }
-
-namespace {
-RegisterXZInterpolation<XZHermiteSpline> registerinterphermitespline{"hermitespline"};
-RegisterXZInterpolation<XZMonotonicHermiteSpline> registerinterpmonotonichermitespline{
-    "monotonichermitespline"};
-RegisterXZInterpolation<XZLagrange4pt> registerinterplagrange4pt{"lagrange4pt"};
-RegisterXZInterpolation<XZBilinear> registerinterpbilinear{"bilinear"};
-} // namespace