How to set igraph edge attributes to strings using a gml format file?

I using GML graph file format to read a graph into igraph (R version). Is there a way to set edge attributes to be strings? It seems that some attribute labels are permitted to have string values while others are not. Sample input file:

graph [
node [
    id 1
    control 1
    label "CiscoSW-1"
]
node [
    id 2
    control 1
    label "CiscoSW-z"
]
edge [
    source 1
    target 2
    difficulty 'A,B,C'
    label "CiscoSW-1"
]
]