Difference between polylines and MultiLineString geometry in QGIS?

Question

After writing some code, I got this error message:

TypeError: MultiLineString geometry cannot be converted to a polyline. Only single line or curve types are permitted.

I wanted to write a script that would iterate over the vertices of what I thought was a "polyline" in QGIS, giving it the name "Polyline1." However, after typing out the following code into the console, I got back the error message that I received above:

import os
layer = QgsProject.instance().mapLayersByName("Polyline1")[0]
features = layer.getFeatures()
for feature in features:
    geom = feature.geometry()
    vertices = geom.asPolyline()
    for vertex in vertices:
        print(vertex.x(), vertex.y())