diff --git a/src/VectorN/finiteVolume/blockMatrixTools/blockMatrixTools.C b/src/VectorN/finiteVolume/blockMatrixTools/blockMatrixTools.C
index a337af777..e329fbef8 100644
--- a/src/VectorN/finiteVolume/blockMatrixTools/blockMatrixTools.C
+++ b/src/VectorN/finiteVolume/blockMatrixTools/blockMatrixTools.C
@@ -1262,7 +1262,7 @@ void insertEquationCoupling
     }
 
     // Get references to fvScalarMatrix fields, updating boundary contributions
-    scalarField& diag = matrix.D();
+    scalarField& diag = matrix.D()();
     scalarField& source = matrix.source();
     matrix.addBoundarySource(source, false);