geometry - Calculate length of selected features in PyQGIS

I create a selection with getFeatures(QgsFeatureRequest()) and I want to calculate the length of the result which is a QgsFeatureIterator :

select = QgsExpression(" myexpression ")
selectedfeats = layer.getFeatures(QgsFeatureRequest(select ))

total = 0
for feature in selectedfeats :
    geom = feature.geometry()
    total += geom.length()

But it doesn't work it returns 0 whereas the selectedfeats is not empty.