Can't integrate normal current density nJ over internal boundary

Something I can't understand. See the attached trivial model. In Surface Integration 1, if I select ec.nJ from the menu, I get an error message (same with trying to plot it or export it), but if I look up the definition of ec.nJ in Equation View of Current Conservation 1, and enter that expression, instead of ec.nJ, it works fine.

ec.nJ is defined as:

sqrt(realdot(ec.Jx,ec.Jx)+realdot(ec.Jy,ec.Jy)+realdot(ec.Jz,ec.Jz))

The error message I get is:


Exception:
com.comsol.nativejni.FlNativeException: Failed to evaluate expression
Messages:
Post expr failed

Failed to evaluate variable
- Variable: mod1.ec.nJ
- Geometry: 1
- Boundary: 6 10 13 15

Failed to evaluate expression
- Expression: real(mod1.ec.nJ)

Failed to evaluate expression
- Expression: real(mod1.ec.nJ)

Stack trace:
at evaltree.cpp, row 708, ()
at xevaluators.cpp, row 1092, ()
at xevaluators.cpp, row 364, ()
at xmodel_post.cpp, row 4408, ()
at com.comsol.nativejni.xmesh.Xmesh.postInt(Native Method)
at com.comsol.nativejni.xmesh.Xmesh.postInt(Unknown Source)
at com.comsol.nativemph.xmesh.Xmesh.a(Unknown Source)
at com.comsol.post.dataset.u.a(Unknown Source)
at com.comsol.post.dataset.ae.a(Unknown Source)
at com.comsol.post.dataset.ab.a(Unknown Source)
at com.comsol.post.dataset.u.a(Unknown Source)
at com.comsol.post.dataset.d.a(Unknown Source)
at com.comsol.post.numerical.e.a(Unknown Source)
at com.comsol.post.numerical.g.a(Unknown Source)
at com.comsol.post.numerical.g.a(Unknown Source)
at com.comsol.post.numerical.g.computeTableData(Unknown Source)
at com.comsol.model.internal.impl.NumericalFeatureImpl.a(Unknown Source)
at com.comsol.model.internal.impl.NumericalFeatureImpl.computeTableData(Unknown Source)
at com.comsol.post.g.h(Unknown Source)
at com.comsol.post.g.b(Unknown Source)
at com.comsol.post.numerical.g.appendResult(Unknown Source)
at com.comsol.model.internal.impl.NumericalFeatureImpl.w(Unknown Source)
at com.comsol.model.internal.impl.NumericalFeatureImpl.g(Unknown Source)
at com.comsol.model.internal.impl.NumericalFeatureImpl$2.a(Unknown Source)
at com.comsol.model.internal.impl.NumericalFeatureImpl$2.execute(Unknown Source)
at com.comsol.model.clientserver.ClientManagerImpl$1.call(Unknown Source)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)