QGIS » QGIS Application

I have a simple vector polygon layer with one feature. Using python I create a simple label displaying the area of the feature.

Expected result:
A square with a label saying "1".

Actual result:
A square without a label.

Try:
Right click the layer and select properties. Click the apply button and the label is now being shown. Since no changes were made in the properties window, I would expect the label not to be shown if this was not a bug. But since the label was shown by pressing the apply button without any changes made to the properties, it seems to me that it is a bug.

Question:
Why is the label not shown using the below code?

Affects:
I have tested the code using QGIS 2.14.7, 2.16.1, master_2 (7396370) on Windows 7 x64, and a recent master_2 on Ubuntu x64.

from qgis.core import *
layer = QgsVectorLayer('Polygon', 'my layer', 'memory')
QgsMapLayerRegistry.instance().addMapLayer(layer, True)
geom = QgsGeometry.fromWkt('POLYGON((0 0, 0 1, 1 1, 1 0, 0 0))')
feature = QgsFeature()
feature.setGeometry(geom)
layer.startEditing()
layer.addFeature(feature)
layer.commitChanges()

label = QgsPalLayerSettings()
label.readFromLayer(layer)
label.enabled = True
label.fieldName = 'round($area,0)'
label.drawLabels = True
label.writeToLayer(layer)