pyqgis - Merging Features

I want to merge features of a vector layer into one single feature using the python console. Somewhere I found the following solution. However it does not seem to work for me. When I execute the following code in the python console

layer = iface.activeLayer()
geoms = QgsGeometry.fromWkt('GEOMETRYCOLLECTION EMPTY')
for feature in layer.getFeatures():
  geoms = geoms.combine(feature.geometry())

I get the error:

Traceback (most recent call last):
  File "", line 2, in 
AttributeError: 'NoneType' object has no attribute 'combine'

What went wrong ? Or how to do it differently with python ?

Answer

Change line 2 to:

geoms = QgsGeometry.fromWkt('GEOMETRYCOLLECTION()')

Then it works, even this version is not conform to Wkt spec, as is your's.