diff --git a/Source/Diagnostics/ReducedDiags/FieldEnergy.cpp b/Source/Diagnostics/ReducedDiags/FieldEnergy.cpp
index 3f81e74c5c2..6040ea4e6dd 100644
--- a/Source/Diagnostics/ReducedDiags/FieldEnergy.cpp
+++ b/Source/Diagnostics/ReducedDiags/FieldEnergy.cpp
@@ -99,7 +99,7 @@ void FieldEnergy::ComputeDiags (int step)
         const MultiFab & Bz = warpx.getField(FieldType::Bfield_aux, lev,2);
 
         // get cell volume
-        const std::array<Real, 3> &dx = warpx.CellSize(lev);
+        const std::array<Real, 3> &dx = WarpX::CellSize(lev);
         const amrex::Real dV = dx[0]*dx[1]*dx[2];
 
         Geometry const & geom = warpx.Geom(lev);
diff --git a/Source/Diagnostics/ReducedDiags/FieldMomentum.cpp b/Source/Diagnostics/ReducedDiags/FieldMomentum.cpp
index 33d3dcca892..7eb16efecff 100644
--- a/Source/Diagnostics/ReducedDiags/FieldMomentum.cpp
+++ b/Source/Diagnostics/ReducedDiags/FieldMomentum.cpp
@@ -184,7 +184,7 @@ void FieldMomentum::ComputeDiags (int step)
         amrex::ParallelDescriptor::ReduceRealSum({ExB_x,ExB_y,ExB_z});
 
         // Get cell volume
-        const std::array<Real, 3> &dx = warpx.CellSize(lev);
+        const std::array<Real, 3> &dx = WarpX::CellSize(lev);
         const amrex::Real dV = dx[0]*dx[1]*dx[2];
 
         // Save data (offset: 3 values for each refinement level)