diff --git a/paimon-core/src/main/java/org/apache/paimon/operation/OrphanFilesClean.java b/paimon-core/src/main/java/org/apache/paimon/operation/OrphanFilesClean.java
index 31d80bf519e8..b7bf5f975065 100644
--- a/paimon-core/src/main/java/org/apache/paimon/operation/OrphanFilesClean.java
+++ b/paimon-core/src/main/java/org/apache/paimon/operation/OrphanFilesClean.java
@@ -109,7 +109,10 @@ public OrphanFilesClean(FileStoreTable table, long olderThanMillis, boolean dryR
     }
 
     protected List<String> validBranches() {
-        List<String> branches = table.branchManager().branches();
+        // The branch manager may return an immutable list (the REST branch manager returns
+        // Collections.emptyList() for a branchless table), so copy it before adding the main
+        // branch.
+        List<String> branches = new ArrayList<>(table.branchManager().branches());
 
         List<String> abnormalBranches = new ArrayList<>();
         for (String branch : branches) {
diff --git a/paimon-core/src/test/java/org/apache/paimon/operation/OrphanFilesCleanTest.java b/paimon-core/src/test/java/org/apache/paimon/operation/OrphanFilesCleanTest.java
index 75648f519bc3..6322f89724d2 100644
--- a/paimon-core/src/test/java/org/apache/paimon/operation/OrphanFilesCleanTest.java
+++ b/paimon-core/src/test/java/org/apache/paimon/operation/OrphanFilesCleanTest.java
@@ -37,6 +37,7 @@
 import org.apache.paimon.types.DataTypes;
 import org.apache.paimon.types.RowKind;
 import org.apache.paimon.types.RowType;
+import org.apache.paimon.utils.BranchManager;
 
 import org.junit.Rule;
 import org.junit.Test;
@@ -44,6 +45,7 @@
 
 import java.io.IOException;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.UUID;
 
@@ -68,6 +70,36 @@ public void testOlderThanMillis() {
                         "The arg olderThan must be less than now, because dataFiles that are currently being written and not referenced by snapshots will be mistakenly cleaned up.");
     }
 
+    @Test
+    public void testValidBranchesDoesNotMutateManagerList() throws Exception {
+        Path tablePath = new Path(tempDir.newFolder().toURI());
+        FileIO fileIO = LocalFileIO.create();
+        RowType rowType = RowType.of(DataTypes.INT(), DataTypes.STRING());
+        TableSchema tableSchema =
+                SchemaUtils.forceCommit(
+                        new FileSystemSchemaManager(fileIO, tablePath),
+                        new Schema(
+                                rowType.getFields(),
+                                Collections.<String>emptyList(),
+                                Collections.<String>emptyList(),
+                                new Options().toMap(),
+                                ""));
+        FileStoreTable real = FileStoreTableFactory.create(fileIO, tablePath, tableSchema);
+
+        // REST catalogs hand out an immutable empty list for branch-less tables; the
+        // cleaner must not try to add the main branch to that shared list
+        BranchManager branchManager = org.mockito.Mockito.mock(BranchManager.class);
+        org.mockito.Mockito.when(branchManager.branches()).thenReturn(Collections.emptyList());
+        FileStoreTable table =
+                org.mockito.Mockito.mock(
+                        FileStoreTable.class, org.mockito.AdditionalAnswers.delegatesTo(real));
+        org.mockito.Mockito.doReturn(branchManager).when(table).branchManager();
+
+        List<String> valid = new LocalOrphanFilesClean(table).validBranches();
+
+        assertThat(valid).containsExactly("main");
+    }
+
     @Test
     public void testListPaimonFileDirsWithEmptyPartition() throws Exception {
         Path tablePath = new Path(tempDir.newFolder().toURI());