diff --git a/paimon-core/src/test/java/org/apache/paimon/mergetree/compact/MergeFunctionTestUtils.java b/paimon-core/src/test/java/org/apache/paimon/mergetree/compact/MergeFunctionTestUtils.java
index 556336bbc451..97410c71717e 100644
--- a/paimon-core/src/test/java/org/apache/paimon/mergetree/compact/MergeFunctionTestUtils.java
+++ b/paimon-core/src/test/java/org/apache/paimon/mergetree/compact/MergeFunctionTestUtils.java
@@ -81,10 +81,18 @@ public static List<ReusingTestData> getExpectedForAggSum(List<ReusingTestData> i
 
         List<ReusingTestData> expected = new ArrayList<>();
         for (List<ReusingTestData> group : groups.values()) {
-            long sum =
-                    group.stream().mapToLong(d -> d.valueKind.isAdd() ? d.value : -d.value).sum();
-            ReusingTestData last = group.get(group.size() - 1);
-            expected.add(new ReusingTestData(last.key, last.sequenceNumber, RowKind.INSERT, sum));
+            if (group.size() == 1) {
+                // due to ReducerMergeFunctionWrapper
+                expected.add(group.get(0));
+            } else {
+                long sum =
+                        group.stream()
+                                .mapToLong(d -> d.valueKind.isAdd() ? d.value : -d.value)
+                                .sum();
+                ReusingTestData last = group.get(group.size() - 1);
+                expected.add(
+                        new ReusingTestData(last.key, last.sequenceNumber, RowKind.INSERT, sum));
+            }
         }
         return expected;
     }