diff --git a/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.cpp b/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.cpp
index fb682bf79e5b..d2c4e14f0664 100644
--- a/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.cpp
+++ b/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.cpp
@@ -134,7 +134,7 @@ bool TSparsedMerger::TPlainChunkCursor::AddIndexTo(const ui32 index, TWriter& wr
 
 bool TSparsedMerger::TSparsedChunkCursor::AddIndexTo(const ui32 index, TWriter& writer) {
     AFL_VERIFY(ChunkStartGlobalPosition <= index);
-    AFL_VERIFY(index == NextGlobalPosition);
+    AFL_VERIFY(index == NextGlobalPosition)("index", index)("next", NextGlobalPosition);
     writer.AddRealData(Chunk->GetColValue(), NextLocalPosition);
     return true;
 }
@@ -163,7 +163,9 @@ void TSparsedMerger::TCursor::InitArrays(const ui32 position) {
         PlainCursor = make_shared<TPlainChunkCursor>(CurrentOwnedArray->GetArray(), &*CurrentOwnedArray);
         SparsedCursor = nullptr;
     }
+    AFL_VERIFY(CurrentOwnedArray->GetAddress().GetGlobalStartPosition() <= position);
     FinishGlobalPosition = CurrentOwnedArray->GetAddress().GetGlobalStartPosition() + CurrentOwnedArray->GetArray()->GetRecordsCount();
+    AFL_VERIFY(position < FinishGlobalPosition);
 }
 
 }   // namespace NKikimr::NOlap::NCompaction
diff --git a/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.h b/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.h
index f9a96d5bf974..9fc64606a092 100644
--- a/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.h
+++ b/ydb/core/tx/columnshard/engines/changes/compaction/sparsed/logic.h
@@ -65,6 +65,8 @@ class TSparsedMerger: public IColumnMerger {
             ChunkStartPosition = CurrentOwnedArray->GetAddress().GetGlobalStartPosition() + ChunkAddress->GetAddress().GetGlobalStartPosition();
             ChunkFinishPosition =
                 CurrentOwnedArray->GetAddress().GetGlobalStartPosition() + ChunkAddress->GetAddress().GetGlobalFinishPosition();
+            AFL_VERIFY(position < ChunkFinishPosition)("finish", ChunkFinishPosition)("pos", position);
+            AFL_VERIFY(ChunkStartPosition <= position)("start", ChunkStartPosition)("pos", position);
         }
 
     public:
@@ -78,7 +80,8 @@ class TSparsedMerger: public IColumnMerger {
         }
         bool AddIndexTo(const ui32 index, TWriter& writer);
         std::optional<ui32> MoveToSignificant(const ui32 currentGlobalPosition, const TColumnMergeContext& context) {
-            AFL_VERIFY(ChunkStartPosition <= currentGlobalPosition);
+            AFL_VERIFY(ChunkStartPosition <= currentGlobalPosition)("start", ChunkStartPosition)("pos", currentGlobalPosition)(
+                "global_start", CurrentOwnedArray->GetAddress().GetGlobalStartPosition());
             ui32 currentIndex = currentGlobalPosition;
             while (true) {
                 if (CurrentOwnedArray->GetAddress().GetGlobalFinishPosition() <= currentIndex) {
@@ -192,6 +195,7 @@ class TSparsedMerger: public IColumnMerger {
                 if (FinishGlobalPosition == Array->GetRecordsCount()) {
                     return FinishGlobalPosition;
                 } else {
+                    currentPosition = FinishGlobalPosition;
                     InitArrays(FinishGlobalPosition);
                 }
             }