diff --git a/src/regionModels/surfaceFilmModels/kinematicSingleLayer/kinematicSingleLayerI.H b/src/regionModels/surfaceFilmModels/kinematicSingleLayer/kinematicSingleLayerI.H
index 89eb8b614543b48de0a1d0814cc14ff06b0b0f57..6a77b614bcad146deb45f56c7546e9d208bbf9ee 100644
--- a/src/regionModels/surfaceFilmModels/kinematicSingleLayer/kinematicSingleLayerI.H
+++ b/src/regionModels/surfaceFilmModels/kinematicSingleLayer/kinematicSingleLayerI.H
@@ -179,7 +179,8 @@ inline tmp<volScalarField> kinematicSingleLayer::netMass() const
 {
     dimensionedScalar d0("SMALL", dimLength, ROOTVSMALL);
     return
-        fvc::surfaceSum(phi_/(fvc::interpolate(delta_) + d0))*time().deltaT()
+        fvc::surfaceSum(pos(phi_)*phi_/(fvc::interpolate(delta_) + d0))
+       *time().deltaT()
       + rho_*delta_*magSf();
 }