diff --git a/src/MiniExcel.OpenXml/Templates/OpenXmlTemplate.Impl.cs b/src/MiniExcel.OpenXml/Templates/OpenXmlTemplate.Impl.cs
index e3933f21..46f565e7 100644
--- a/src/MiniExcel.OpenXml/Templates/OpenXmlTemplate.Impl.cs
+++ b/src/MiniExcel.OpenXml/Templates/OpenXmlTemplate.Impl.cs
@@ -49,6 +49,7 @@ private async Task GenerateSheetByUpdateModeAsync(ZipArchiveEntry sheetZipEntry,
         var sheetData = worksheet?.Element(SpreadsheetNs + "sheetData");
         var newSheetData = new XElement(sheetData);
         var rows = newSheetData.Elements(SpreadsheetNs + "row");
+        FillMissingReferences(rows);
 
         InjectSharedStrings(sharedStrings, rows);
         GetMergeCells(worksheet);
@@ -83,6 +84,7 @@ private async Task GenerateSheetByCreateModeAsync(ZipArchiveEntry templateSheetZ
         var newSheetData = new XElement(sheetData);
     
         var rows = newSheetData.Elements(SpreadsheetNs + "row");
+        FillMissingReferences(rows);
 
         InjectSharedStrings(sharedStrings, rows);
         GetMergeCells(worksheet);
@@ -97,6 +99,39 @@ private async Task GenerateSheetByCreateModeAsync(ZipArchiveEntry templateSheetZ
         await WriteSheetXmlAsync(writer, worksheet, sheetData, mergeCells, cancellationToken).ConfigureAwait(false);
     }
 
+    // "r" is optional on rows and cells (ECMA-376 18.3.1.73, 18.3.1.4); without it they follow the previous one,
+    // which is also how OpenXmlReader reads them. The rest of the template code keys on "r", so fill it in first.
+    private static void FillMissingReferences(IEnumerable<XElement> rows)
+    {
+        var rowIndex = 0;
+        foreach (var row in rows)
+        {
+            if (row.Attribute("r") is not { } rowReference)
+            {
+                rowIndex++;
+                row.SetAttributeValue("r", rowIndex.ToString());
+            }
+            else if (int.TryParse(rowReference.Value, out var explicitRowIndex))
+            {
+                rowIndex = explicitRowIndex;
+            }
+
+            var columnIndex = 0;
+            foreach (var cell in row.Elements(SpreadsheetNs + "c"))
+            {
+                if (cell.Attribute("r") is not { } cellReference)
+                {
+                    columnIndex++;
+                    cell.SetAttributeValue("r", CellReferenceConverter.GetCellFromCoordinates(columnIndex, rowIndex));
+                }
+                else if (CellReferenceConverter.TryParseCellReference(cellReference.Value, out var explicitColumnIndex, out _))
+                {
+                    columnIndex = explicitColumnIndex;
+                }
+            }
+        }
+    }
+
     private void GetMergeCells(XElement worksheet)
     {
         if (worksheet.Element(SpreadsheetNs + "mergeCells") is not { } mergeCells)
@@ -334,7 +369,6 @@ var s when s.StartsWith("@header") => SpecialCellType.Header,
                 }
             }
 
-            //TODO: Fix parsing for documents that don't have the "r" attribute on rows
             if (row.Attribute("r")?.Value is not { } rVal || !int.TryParse(rVal, out var originRowIndex))
                 throw new NotSupportedException("The format of the chosen template is not currently supported.");
 
diff --git a/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesAsyncTests.cs b/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesAsyncTests.cs
index 75d26f9e..bf85ff61 100644
--- a/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesAsyncTests.cs
+++ b/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesAsyncTests.cs
@@ -1313,4 +1313,30 @@ public async Task TestIssue980() // deterministic file generation
         var bytes2 = await File.ReadAllBytesAsync(path2.FilePath); 
         Assert.True(bytes1.SequenceEqual(bytes2));
     }
+
+    [Fact]
+    public async Task TestIssue863() // template rows and cells without the optional "r" reference
+    {
+        var original = PathHelper.GetFile("xlsx/TestTemplateComplex.xlsx");
+        using var withoutReferences = AutoDeletingPath.Create();
+        SheetHelper.CopyWithoutCellReferences(original, withoutReferences.FilePath);
+
+        var value = new Dictionary<string, object>
+        {
+            ["title"] = "FooCompany",
+            ["managers"] = new[] { new { name = "Jack", department = "HR" }, new { name = "Loan", department = "IT" } },
+            ["employees"] = new[] { new { name = "Wade", department = "HR" }, new { name = "Keaton", department = "IT" } }
+        };
+
+        using var expected = AutoDeletingPath.Create();
+        using var actual = AutoDeletingPath.Create();
+        await _excelTemplater.FillTemplateAsync(expected.FilePath, original, value);
+        await _excelTemplater.FillTemplateAsync(actual.FilePath, withoutReferences.FilePath, value);
+
+        var expectedRows = _excelImporter.Query(expected.FilePath).Select(row => ((IDictionary<string, object?>)row).Values.ToArray()).ToList();
+        var actualRows = _excelImporter.Query(actual.FilePath).Select(row => ((IDictionary<string, object?>)row).Values.ToArray()).ToList();
+        Assert.Equal("A1:C7", SheetHelper.GetFirstSheetDimensionRefValue(actual.FilePath));
+        Assert.Equal(expectedRows, actualRows);
+        Assert.Equal("Keaton", actualRows[6][1]);
+    }
 }
diff --git a/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesTests.cs b/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesTests.cs
index bc95e98a..56b4d853 100644
--- a/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesTests.cs
+++ b/tests/MiniExcel.OpenXml.Tests/Issues/MiniExcelGithubIssuesTests.cs
@@ -3029,4 +3029,49 @@ public void TestIssue980() // deterministic file generation
         var bytes2 = File.ReadAllBytes(path2.FilePath); 
         Assert.True(bytes1.SequenceEqual(bytes2));
     }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void TestIssue863(bool keepRowReferences) // template rows and cells without the optional "r" reference
+    {
+        var original = PathHelper.GetFile("xlsx/TestTemplateComplex.xlsx");
+        using var withoutReferences = AutoDeletingPath.Create();
+        SheetHelper.CopyWithoutCellReferences(original, withoutReferences.FilePath, keepRowReferences);
+
+        var value = new Dictionary<string, object>
+        {
+            ["title"] = "FooCompany",
+            ["managers"] = new[] { new { name = "Jack", department = "HR" }, new { name = "Loan", department = "IT" } },
+            ["employees"] = new[] { new { name = "Wade", department = "HR" }, new { name = "Keaton", department = "IT" } }
+        };
+
+        using var expected = AutoDeletingPath.Create();
+        using var actual = AutoDeletingPath.Create();
+        _excelTemplater.FillTemplate(expected.FilePath, original, value);
+        _excelTemplater.FillTemplate(actual.FilePath, withoutReferences.FilePath, value);
+
+        var actualRows = ReadValues(actual.FilePath);
+        Assert.Equal("A1:C7", SheetHelper.GetFirstSheetDimensionRefValue(actual.FilePath));
+        Assert.Equal(ReadValues(expected.FilePath), actualRows);
+        Assert.Equal("Keaton", actualRows[6][1]);
+    }
+
+    [Fact]
+    public void TestIssue863_MergeSameCells()
+    {
+        using var withoutReferences = AutoDeletingPath.Create();
+        SheetHelper.CopyWithoutCellReferences(PathHelper.GetFile("xlsx/TestMergeWithTag.xlsx"), withoutReferences.FilePath);
+        using var merged = AutoDeletingPath.Create();
+
+        _excelTemplater.MergeSameCells(merged.FilePath, withoutReferences.FilePath);
+
+        var mergedCells = SheetHelper.GetFirstSheetMergedCells(merged.FilePath);
+        Assert.Equal("A2:A4", mergedCells[0]);
+        Assert.Equal("C3:C4", mergedCells[1]);
+        Assert.Equal("A7:A8", mergedCells[2]);
+    }
+
+    private List<object?[]> ReadValues(string path) =>
+        _excelImporter.Query(path).Select(row => ((IDictionary<string, object?>)row).Values.ToArray()).ToList();
 }
diff --git a/tests/MiniExcel.OpenXml.Tests/Utils/SheetHelper.cs b/tests/MiniExcel.OpenXml.Tests/Utils/SheetHelper.cs
index 056bd99c..26cc2961 100644
--- a/tests/MiniExcel.OpenXml.Tests/Utils/SheetHelper.cs
+++ b/tests/MiniExcel.OpenXml.Tests/Utils/SheetHelper.cs
@@ -133,4 +133,25 @@ internal static MemoryStream CreateTestWorkbookStream()
         stream.Position = 0; 
         return stream;
     }
+
+    // Copy of the workbook without "r" on its cells, and on its rows unless keepRowReferences (ECMA-376 allows both)
+    internal static void CopyWithoutCellReferences(string source, string target, bool keepRowReferences = false)
+    {
+        File.Copy(source, target, overwrite: true);
+        using var zip = ZipFile.Open(target, ZipArchiveMode.Update);
+        foreach (var entry in zip.Entries.Where(e => e.FullName.StartsWith("xl/worksheets/sheet")).ToList())
+        {
+            XDocument doc;
+            using (var stream = entry.Open())
+                doc = XDocument.Load(stream);
+
+            foreach (var element in doc.Descendants().Where(e => e.Name.LocalName == "c" || (e.Name.LocalName == "row" && !keepRowReferences)))
+                element.Attribute("r")?.Remove();
+
+            var name = entry.FullName;
+            entry.Delete();
+            using var output = zip.CreateEntry(name).Open();
+            doc.Save(output);
+        }
+    }
 }