Single quoted strings not supported?

[binarycow]

Using a single quoted string results in errors

module alpha {
    namespace 'uri:alpha';
    prefix a;
    yang-version 1.1;
}

Errors produced:

• <data def stmt>, <linkage stmts>, <meta stmts>, augment, deviation, extension, feature, grouping, identity, notification, revision, rpc or typedef expected, got '''
• <line break>, <uri str>, <wsp>, YangTokenType.DOUBLE_QUOTE, YangTokenType.FORWARD_SLASH or YangTokenType.SINGLE_QUOTE expected, got '''

This works with no issues in pyang. Additionally, the errors go away if I use a double quoted string for the namespace.