bootstrap-theme.css.map not generated by Web Essentials - visual-studio

Question

When Web Essentials minifies css files, it does not generate the map files.
Do I miss any special settings?
Here's my WebEssentials-Settings.json:
{
"BrowserLink": {
"CssIgnorePatterns": "bootstrap*; reset.css; normalize.css; jquery*; toastr*; foundation*; animate*; inuit*; elements*; ratchet*; hint*; flat-ui*; 960*; skeleton*",
"EnableMenu": true,
"EnablePixelPushing": true,
"ShowMenu": true
},
"CodeGen": {
"CamelCasePropertyNames": true,
"CamelCaseTypeNames": false
},
"CoffeeScript": {
"CompileOnBuild": false,
"CompileOnSave": true,
"GenerateSourceMaps": true,
"LintOnBuild": false,
"LintOnSave": true,
"LintResultLocation": "Message",
"MinifyInPlace": false,
"OutputDirectory": null,
"ShowPreviewPane": true,
"WrapClosure": true
},
"Css": {
"AdjustRelativePaths": true,
"AutoMinify": true,
"GzipMinifiedFiles": false,
"ShowBrowserTooltip": true,
"ShowInitialInherit": false,
"ShowUnsupported": true,
"SyncVendorValues": true,
"ValidateEmbedImages": true,
"ValidateOverQualifiedSelector": true,
"ValidateStarSelector": true,
"ValidateVendorSpecifics": true,
"ValidateZeroUnit": true,
"ValidationLocation": "Messages"
},
"General": {
"AllMessagesToOutputWindow": false,
"KeepImportantComments": true,
"SvgPreviewPane": true
},
"Html": {
"AutoMinify": false,
"EnableAngularValidation": true,
"EnableEnterFormat": true,
"GzipMinifiedFiles": false,
"ImageDropFormats": [
{
"HtmlFormat": "<img src=\"{0}\" alt=\"\" />",
"Name": "Simple Image Tag"
},
{
"HtmlFormat": "<div><img src=\"{0}\" alt=\"\" /></div>",
"Name": "Enclosed in Div"
},
{
"HtmlFormat": "<li id=\"item_{1}\"><img src=\"{0}\" alt=\"\" /></li>",
"Name": "Enclosed as List Item"
},
{
"HtmlFormat": "<div style=\"background-image=url('{0}')\"></div>",
"Name": "Inline CSS"
}
]
},
"JavaScript": {
"AutoMinify": true,
"BlockCommentCompletion": true,
"GenerateSourceMaps": true,
"GzipMinifiedFiles": false,
"LintOnBuild": false,
"LintOnSave": true,
"LintResultLocation": "Message"
},
"Less": {
"CompileOnBuild": false,
"CompileOnSave": true,
"EnableChainCompilation": true,
"GenerateSourceMaps": true,
"MinifyInPlace": false,
"OutputDirectory": null,
"ShowPreviewPane": true
},
"Markdown": {
"AutoHyperlink": false,
"AutoNewLines": false,
"CompileOnBuild": false,
"CompileOnSave": true,
"EncodeProblemUrlCharacters": false,
"GenerateXHTML": true,
"LinkEmails": false,
"MinifyInPlace": false,
"OutputDirectory": null,
"ShowPreviewPane": true,
"StrictBoldItalic": false
},
"Scss": {
"CompileOnBuild": false,
"CompileOnSave": true,
"EnableChainCompilation": true,
"GenerateSourceMaps": true,
"MinifyInPlace": false,
"OutputDirectory": null,
"ShowPreviewPane": true
},
"SweetJs": {
"CompileOnBuild": false,
"CompileOnSave": true,
"GenerateSourceMaps": true,
"MinifyInPlace": false,
"OutputDirectory": null,
"ShowPreviewPane": true
},
"TypeScript": {
"LintOnBuild": false,
"LintOnSave": true,
"LintResultLocation": "Message",
"ShowPreviewPane": true
}
}