diff --git a/src/map/style/mapcssparser.cpp b/src/map/style/mapcssparser.cpp
index fd0971a..e3365d5 100644
--- a/src/map/style/mapcssparser.cpp
+++ b/src/map/style/mapcssparser.cpp
@@ -1,131 +1,131 @@
 /*
     Copyright (C) 2020 Volker Krause <vkrause@kde.org>
 
     This program is free software; you can redistribute it and/or modify it
     under the terms of the GNU Library General Public License as published by
     the Free Software Foundation; either version 2 of the License, or (at your
     option) any later version.
 
     This program is distributed in the hope that it will be useful, but WITHOUT
     ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
     FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
     License for more details.
 
     You should have received a copy of the GNU General Public License
     along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 
 #include "mapcssparser.h"
 
 #include "mapcssparser_p.h"
 #include "mapcssscanner.h"
 #include "mapcssstyle.h"
 
 #include <QDebug>
 #include <QFile>
 #include <QFileInfo>
 #include <QScopeGuard>
 
 #include <cstring>
 
 char* unquoteString(const char *str)
 {
     const auto size = strlen(str) - 2;
     if (size <= 0)
         return nullptr;
     auto out = (char*)malloc(size + 1);
     memset(out, 0, size + 1);
     auto outIt = out;
     for (auto it = str + 1; it < str + size + 1; ++it, ++outIt) {
         if (*it == '\\') {
             ++it;
             switch (*it) {
                 case '\\':
                 case '"':
                     *outIt = *it; break;
                 case 'n':
                     *outIt = '\n'; break;
                 case 't':
                     *outIt = '\t'; break;
                 default:
                     *outIt++ = '\\';
                     *outIt = *it;
             }
         } else {
             *outIt = *it;
         }
     }
     return out;
 }
 
 using namespace KOSMIndoorMap;
 
 bool MapCSSParser::hasError() const
 {
     return m_error;
 }
 
 QString MapCSSParser::fileName() const
 {
     return m_currentFileName;
 }
 
 MapCSSStyle MapCSSParser::parse(const QString &fileName)
 {
     m_error = true;
 
     MapCSSStyle style;
     parse(&style, fileName);
     if (m_error) {
-        return {};
+        return MapCSSStyle();
     }
 
     return style;
 }
 
 void MapCSSParser::parse(MapCSSStyle *style, const QString &fileName)
 {
     QFile f(fileName);
     if (!f.open(QFile::ReadOnly)) {
         qWarning() << f.fileName() << f.errorString();
         return;
     }
     m_currentFileName = fileName;
     m_currentStyle = style;
 
     yyscan_t scanner;
     if (yylex_init(&scanner)) {
         return;
     }
     const auto lexerCleanup = qScopeGuard([&scanner]{ yylex_destroy(scanner); });
 
     const auto b = f.readAll();
     YY_BUFFER_STATE state;
     state = yy_scan_string(b.constData(), scanner);
     if (yyparse(this, scanner)) {
         return;
     }
 
     yy_delete_buffer(state, scanner);
 
     m_error = false;
     m_currentStyle = nullptr;
 }
 
 void MapCSSParser::addImport(char* fileName)
 {
     auto cssFile = QString::fromUtf8(fileName);
     free(fileName);
 
     if (QFileInfo(cssFile).isRelative()) {
         cssFile = QFileInfo(m_currentFileName).absolutePath() + QLatin1Char('/') + cssFile;
     }
 
     MapCSSParser p;
     p.parse(m_currentStyle, cssFile);
 }
 
 void MapCSSParser::addRule(MapCSSRule *rule)
 {
     m_currentStyle->m_rules.push_back(std::unique_ptr<MapCSSRule>(rule));
 }
diff --git a/src/map/style/mapcssstyle.h b/src/map/style/mapcssstyle.h
index 60bd3e0..f6eb001 100644
--- a/src/map/style/mapcssstyle.h
+++ b/src/map/style/mapcssstyle.h
@@ -1,77 +1,80 @@
 /*
     Copyright (C) 2020 Volker Krause <vkrause@kde.org>
 
     This program is free software; you can redistribute it and/or modify it
     under the terms of the GNU Library General Public License as published by
     the Free Software Foundation; either version 2 of the License, or (at your
     option) any later version.
 
     This program is distributed in the hope that it will be useful, but WITHOUT
     ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
     FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
     License for more details.
 
     You should have received a copy of the GNU General Public License
     along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 
 #ifndef KOSMINDOORMAP_MAPCSSSTYLE_H
 #define KOSMINDOORMAP_MAPCSSSTYLE_H
 
 #include "mapcssrule.h"
 
 #include <osm/element.h>
 
 #include <memory>
 #include <vector>
 
 class QIODevice;
 
 namespace OSM {
 class DataSet;
 }
 
 namespace KOSMIndoorMap {
 
 class MapCSSParser;
 class MapCSSResult;
 
-/** A parsed MapCSS style sheet. */
+/** A parsed MapCSS style sheet.
+ *  @see MapCSSParser::parse for how to obtain a valid instance
+ */
 class MapCSSStyle
 {
 public:
+    /** Creates an invalid/empty style. */
+    explicit MapCSSStyle();
     MapCSSStyle(const MapCSSStyle&) = delete;
     MapCSSStyle(MapCSSStyle&&);
     ~MapCSSStyle();
 
+    MapCSSStyle& operator=(const MapCSSStyle&) = delete;
     MapCSSStyle& operator=(MapCSSStyle&&);
 
     /** Optimizes style sheet rules for application against @p dataSet.
      *  This does resolve tag keys and is therefore mandatory when changing the data set.
      */
     void compile(const OSM::DataSet &dataSet);
 
     /** Evaluates the style sheet for a given state @p state (OSM element, view state, element state, etc).
      *  The result is not returned but added to @p result for reusing allocated memory
      *  between evaluations.
      */
     void evaluate(const MapCSSState &state, MapCSSResult &result) const;
 
     /** Evaluate canvas style rules. */
     void evaluateCanvas(MapCSSResult &result) const;
 
     /** Write this style as MapCSS to @p out.
      *  Mainly used for testing.
      */
     void write(QIODevice *out) const;
 
 private:
     friend class MapCSSParser;
-    MapCSSStyle();
-
     std::vector<std::unique_ptr<MapCSSRule>> m_rules;
 };
 
 }
 
 #endif // KOSMINDOORMAP_MAPCSSSTYLE_H