Import with media query and sourcemap enabled emits error

Hello!
When compiling using lessc --source-map-map-inline index.less command following files

index.less

.test {
    color: #000;
    @import "in.less" only screen and (min-width: 1024);
}

in.less

a {
    color: #ccc;
}

Error emitted by lessc:

TypeError: Cannot read property 'substring' of undefined
    at SourceMapOutput.add (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\source-map-output.js:67:38)
    at Element.genCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\tree\element.js:35:12)
    at Selector.genCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\tree\selector.js:103:21)
    at Ruleset.genCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\tree\ruleset.js:386:21)
    at Directive.outputRuleset (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\tree\directive.js:138:18)
    at Media.genCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\tree\media.js:32:10)
    at Ruleset.genCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\tree\ruleset.js:411:18)
    at SourceMapOutput.toCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\source-map-output.js:118:24)
    at SourceMapBuilder.toCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\source-map-builder.js:23:35)
    at ParseTree.toCSS (C:\Users\Fire\AppData\Roaming\npm\node_modules\less\lib\less\parse-tree.js:33:47)

Without --source-map-map-inline parameter compilation runs normally.

Last not affected version is 2.4.0.

Reproduce the issue with the shown index.less and in.less inputs using lessc --source-map-map-inline. Start by reading lib/less/source-map-output.js around SourceMapOutput.add and the related calls in tree/element.js and tree/media.js. Done means the media-query import compiles successfully with an inline sourcemap, while the existing normal compilation behavior remains intact.