Index: /trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSStyleSource.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSStyleSource.java	(revision 7123)
+++ /trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSStyleSource.java	(revision 7124)
@@ -10,4 +10,5 @@
 import java.io.InputStream;
 import java.nio.charset.StandardCharsets;
+import java.text.MessageFormat;
 import java.util.ArrayList;
 import java.util.List;
@@ -126,5 +127,6 @@
             }
             MapCSSRule optRule = new MapCSSRule(r.selector.optimizedBaseCheck(), r.declaration);
-            switch (((GeneralSelector) selRightmost).getBase()) {
+            final String base = ((GeneralSelector) selRightmost).getBase();
+            switch (base) {
                 case "node":
                     nodeRules.add(optRule);
@@ -149,4 +151,9 @@
                 case "canvas":
                     canvasRules.add(r);
+                default:
+                    final RuntimeException e = new RuntimeException(MessageFormat.format("Unknown MapCSS base selector {0}", base));
+                    Main.warn(tr("Failed to parse Mappaint styles from ''{0}''. Error was: {1}", url, e.getMessage()));
+                    Main.error(e);
+                    logError(e);
             }
         }
Index: /trunk/test/unit/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSParserTest.groovy
===================================================================
--- /trunk/test/unit/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSParserTest.groovy	(revision 7123)
+++ /trunk/test/unit/org/openstreetmap/josm/gui/mappaint/mapcss/MapCSSParserTest.groovy	(revision 7124)
@@ -310,3 +310,11 @@
         assert !s1.matches(new Environment().withPrimitive(w))
     }
+
+    @Test
+    public void testInvalidBaseSelector() throws Exception {
+        def css = new MapCSSStyleSource("invalid_base[key=value] {}")
+        css.loadStyleSource()
+        assert !css.getErrors().isEmpty()
+        assert css.getErrors().iterator().next().toString().contains("Unknown MapCSS base selector invalid_base")
+    }
 }