Index: /trunk/src/org/openstreetmap/josm/actions/RestorePropertyAction.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/actions/RestorePropertyAction.java	(revision 17995)
+++ /trunk/src/org/openstreetmap/josm/actions/RestorePropertyAction.java	(revision 17996)
@@ -6,8 +6,8 @@
 import java.awt.event.ActionEvent;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.Map;
 import java.util.function.IntFunction;
 import java.util.function.Supplier;
-import java.util.stream.Collectors;
 
 import javax.swing.AbstractAction;
@@ -56,5 +56,5 @@
 
         Map<String, String> changes = TableHelper.selectedIndices(selectionModel).boxed()
-                .collect(Collectors.toMap(keyFn::apply, valueFn::apply, (a, b) -> b));
+                .collect(HashMap::new, (m,i)->m.put(keyFn.apply(i), valueFn.apply(i)), HashMap::putAll);
         ChangePropertyCommand command = new ChangePropertyCommand(Collections.singleton(primitive), changes);
         UndoRedoHandler.getInstance().add(command);
Index: /trunk/test/unit/org/openstreetmap/josm/actions/RestorePropertyActionTest.java
===================================================================
--- /trunk/test/unit/org/openstreetmap/josm/actions/RestorePropertyActionTest.java	(revision 17996)
+++ /trunk/test/unit/org/openstreetmap/josm/actions/RestorePropertyActionTest.java	(revision 17996)
@@ -0,0 +1,49 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.actions;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import javax.swing.DefaultListSelectionModel;
+import javax.swing.ListSelectionModel;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+import org.openstreetmap.josm.command.ChangePropertyCommand;
+import org.openstreetmap.josm.data.UndoRedoHandler;
+import org.openstreetmap.josm.data.coor.LatLon;
+import org.openstreetmap.josm.data.osm.DataSet;
+import org.openstreetmap.josm.data.osm.Node;
+import org.openstreetmap.josm.testutils.JOSMTestRules;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+/**
+ * Unit tests of {@link RestorePropertyAction}
+ */
+class RestorePropertyActionTest {
+
+    /**
+     * Setup test.
+     */
+    @RegisterExtension
+    @SuppressFBWarnings(value = "URF_UNREAD_PUBLIC_OR_PROTECTED_FIELD")
+    public JOSMTestRules test = new JOSMTestRules().preferences();
+
+    @Test
+    void testTicket20965() {
+        doTest20965(null, null);
+        doTest20965("key", null);
+        doTest20965(null, "val");
+        doTest20965("key", "val");
+    }
+
+    private static void doTest20965(String key, String val) {
+        ListSelectionModel selModel = new DefaultListSelectionModel();
+        selModel.setSelectionInterval(1, 2);
+        Node n = new Node(LatLon.NORTH_POLE);
+        new DataSet(n);
+        UndoRedoHandler.getInstance().clean();
+        new RestorePropertyAction(k -> key, v -> val, () -> n, selModel).actionPerformed(null);
+        assertTrue(UndoRedoHandler.getInstance().getLastCommand() instanceof ChangePropertyCommand);
+    }
+}