diff --git a/src/main/java/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTest.java b/src/main/java/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTest.java
index 25fa351..cb3a325 100644
--- a/src/main/java/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTest.java
+++ b/src/main/java/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTest.java
@@ -4,6 +4,9 @@ package org.openstreetmap.josm.plugins.mapwithai.data.validation.tests;
 import static org.openstreetmap.josm.tools.I18n.marktr;
 import static org.openstreetmap.josm.tools.I18n.tr;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -203,7 +206,7 @@ public class RoutingIslandsTest extends Test {
             if (lastNode != null && lastNode.isOutsideDownloadArea()) {
                 outgoingWays.add(way);
             }
-            if (isOneway == 0 && firstNode != null && lastNode != null
+            if (isOneway != null && isOneway == 0 && firstNode != null && lastNode != null
                     && (firstNode.isOutsideDownloadArea() || lastNode.isOutsideDownloadArea())) {
                 incomingWays.add(way);
                 outgoingWays.add(way);
@@ -301,7 +304,8 @@ public class RoutingIslandsTest extends Test {
                 Set<Way> referrers = node.getReferrers(true).parallelStream().filter(Way.class::isInstance)
                         .map(Way.class::cast).filter(tWay -> !directional.contains(tWay)).collect(Collectors.toSet());
                 for (Way tWay : referrers) {
-                    if (isOneway(tWay, currentTransportMode) == 0 || predicate.test(tWay, way) || tWay.isClosed()) {
+                    Integer oneWay = isOneway(tWay, currentTransportMode);
+                    if ((oneWay != null && oneWay == 0) || predicate.test(tWay, way) || tWay.isClosed()) {
                         toAdd.add(tWay);
                     }
                 }
@@ -363,7 +367,8 @@ public class RoutingIslandsTest extends Test {
      * @return See {@link Way#isOneway} (but may additionally return {@code null} if
      *         the transport type cannot route down that way)
      */
-    public static Integer isOneway(Way way, String transportType) {
+    @Nullable
+    public static Integer isOneway(@Nonnull Way way, @Nullable String transportType) {
         if (transportType == null || transportType.trim().isEmpty()) {
             return way.isOneway();
         }
diff --git a/test/unit/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTestTest.java b/test/unit/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTestTest.java
index 531a9da..115e975 100644
--- a/test/unit/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTestTest.java
+++ b/test/unit/org/openstreetmap/josm/plugins/mapwithai/data/validation/tests/RoutingIslandsTestTest.java
@@ -1,6 +1,7 @@
 // License: GPL. For details, see LICENSE file.
 package org.openstreetmap.josm.plugins.mapwithai.data.validation.tests;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
@@ -12,8 +13,8 @@ import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
 import java.util.stream.Collectors;
+import java.util.stream.DoubleStream;
 
-import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -26,13 +27,18 @@ import org.openstreetmap.josm.data.osm.DataSet;
 import org.openstreetmap.josm.data.osm.Node;
 import org.openstreetmap.josm.data.osm.OsmPrimitive;
 import org.openstreetmap.josm.data.osm.Way;
+import org.openstreetmap.josm.data.preferences.sources.ValidatorPrefHelper;
 import org.openstreetmap.josm.data.validation.Severity;
+import org.openstreetmap.josm.gui.progress.NullProgressMonitor;
 import org.openstreetmap.josm.plugins.mapwithai.testutils.annotations.MapWithAISources;
 import org.openstreetmap.josm.plugins.mapwithai.testutils.annotations.NoExceptions;
+import org.openstreetmap.josm.plugins.mapwithai.tools.Access;
 import org.openstreetmap.josm.spi.preferences.Config;
 import org.openstreetmap.josm.testutils.JOSMTestRules;
 import org.openstreetmap.josm.testutils.annotations.BasicPreferences;
 
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
 /**
  * Test class for {@link RoutingIslandsTest}
  *
@@ -240,6 +246,34 @@ class RoutingIslandsTestTest {
         assertTrue(outgoingSet.parallelStream().allMatch(way -> Arrays.asList(way1, way2).contains(way)));
     }
 
+    /**
+     * Non-regression test for #21551
+     */
+    @Test
+    void testNonRegression21551() {
+        ValidatorPrefHelper.PREF_OTHER.put(Boolean.TRUE);
+        final DataSet dataSet = new DataSet();
+        dataSet.addDataSource(new DataSource(new Bounds(-18, -180, 18, 180), "World bounds testNonRegression21551"));
+        final Way way1 = new Way();
+        way1.setNodes(DoubleStream.iterate(0, d -> d + 1).limit(20).mapToObj(d -> new LatLon(d, d)).map(Node::new)
+                .collect(Collectors.toList()));
+        final Way way2 = new Way();
+        way2.setNodes(DoubleStream.iterate(-1, d -> d - 1).limit(19).mapToObj(d -> new LatLon(d, d)).map(Node::new)
+                .collect(Collectors.toList()));
+        way2.addNode(way1.firstNode());
+        addToDataSet(dataSet, way1);
+        addToDataSet(dataSet, way2);
+        way1.put("highway", "residential");
+        way2.put("highway", "residential");
+        way2.put(Access.AccessTags.MOTOR_VEHICLE.getKey() + ":forward", Access.AccessTags.NO.getKey());
+        way2.put(Access.AccessTags.MOTOR_VEHICLE.getKey() + ":backward", Access.AccessTags.NO.getKey());
+        final RoutingIslandsTest test = new RoutingIslandsTest();
+        test.startTest(NullProgressMonitor.INSTANCE);
+        test.visit(way1);
+        test.visit(way2);
+        assertDoesNotThrow(test::endTest);
+    }
+
     /**
      * Test method for {@link RoutingIslandsTest#outsideConnections(Node)}.
      */