Index: trunk/src/org/openstreetmap/josm/io/GeoJSONReader.java
===================================================================
--- trunk/src/org/openstreetmap/josm/io/GeoJSONReader.java	(revision 17354)
+++ trunk/src/org/openstreetmap/josm/io/GeoJSONReader.java	(revision 17356)
@@ -45,4 +45,5 @@
 import org.openstreetmap.josm.gui.progress.NullProgressMonitor;
 import org.openstreetmap.josm.gui.progress.ProgressMonitor;
+import org.openstreetmap.josm.tools.CheckParameterUtil;
 import org.openstreetmap.josm.tools.Logging;
 import org.openstreetmap.josm.tools.Utils;
@@ -85,4 +86,6 @@
                 .orElseThrow(() -> new IllegalDataException("No type")).getString()) {
             case "FeatureCollection":
+                JsonValue.ValueType valueType = object.get(FEATURES).getValueType();
+                CheckParameterUtil.ensureThat(valueType == JsonValue.ValueType.ARRAY, "features must be ARRAY, but is " + valueType);
                 parseFeatureCollection(object.getJsonArray(FEATURES));
                 break;