diff --git a/src/SixLabors.Fonts/GlyphShapingData.cs b/src/SixLabors.Fonts/GlyphShapingData.cs
index 042bfdbf..f5b9f2f2 100644
--- a/src/SixLabors.Fonts/GlyphShapingData.cs
+++ b/src/SixLabors.Fonts/GlyphShapingData.cs
@@ -17,6 +17,7 @@ internal class GlyphShapingData
     /// <summary>
     /// Initializes a new instance of the <see cref="GlyphShapingData"/> class.
     /// </summary>
+    /// <param name="textRun">The text run.</param>
     public GlyphShapingData(TextRun textRun) => this.TextRun = textRun;
 
     /// <summary>
diff --git a/src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/UniversalShaper.cs b/src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/UniversalShaper.cs
index 5a55edec..d2575439 100644
--- a/src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/UniversalShaper.cs
+++ b/src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/UniversalShaper.cs
@@ -143,7 +143,7 @@ private static void SetupSyllables(IGlyphShapingCollection collection, int index
             }
 
             // Assign rphf feature
-            int limit = substitutionCollection[match.StartIndex].UniversalShapingEngineInfo!.Category == "R"
+            int limit = substitutionCollection[match.StartIndex + index].UniversalShapingEngineInfo!.Category == "R"
                 ? 1
                 : Math.Min(3, match.EndIndex - match.StartIndex);
 
diff --git a/tests/SixLabors.Fonts.Tests/Issues/Issues_388_390.cs b/tests/SixLabors.Fonts.Tests/Issues/Issues_388_390.cs
new file mode 100644
index 00000000..d60f3401
--- /dev/null
+++ b/tests/SixLabors.Fonts.Tests/Issues/Issues_388_390.cs
@@ -0,0 +1,44 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+#if OS_WINDOWS
+using SixLabors.Fonts.Unicode;
+
+namespace SixLabors.Fonts.Tests.Issues;
+public class Issues_388_390
+{
+    [Fact]
+    public void UniversalShaper_NullReferenceException_388()
+    {
+        CodePoint taiLeCharacter = new(0x195C);     // ᥜ
+        CodePoint sundaneseCharacter = new(0x1B9B); // ᮛ
+        CodePoint tifinaghCharacter = new(0x2D43);  // ⵃ
+        CodePoint chamCharacter = new(0xAA43);      // ꩃ
+
+        CodePoint latainCharacter = new(0x0041);    // A
+        CodePoint hiraganaCharacter = new(0x3042);  // あ
+
+        FontFamily fontFamily = SystemFonts.Get("Yu Gothic");
+        Font font = fontFamily.CreateFont(20.0F);
+        TextOptions textOption = new(font);
+
+        _ = TextMeasurer.MeasureBounds($"{latainCharacter}{taiLeCharacter}", textOption);
+        _ = TextMeasurer.MeasureBounds($"{hiraganaCharacter}{taiLeCharacter}", textOption);
+        _ = TextMeasurer.MeasureBounds($"{latainCharacter}{sundaneseCharacter}", textOption);
+        _ = TextMeasurer.MeasureBounds($"{hiraganaCharacter}{sundaneseCharacter}", textOption);
+        _ = TextMeasurer.MeasureBounds($"{latainCharacter}{tifinaghCharacter}", textOption);
+        _ = TextMeasurer.MeasureBounds($"{hiraganaCharacter}{tifinaghCharacter}", textOption);
+        _ = TextMeasurer.MeasureBounds($"{latainCharacter}{chamCharacter}", textOption);
+        _ = TextMeasurer.MeasureBounds($"{hiraganaCharacter}{chamCharacter}", textOption);
+    }
+
+    [Fact]
+    public void UniversalShaper_NullReferenceException_390()
+    {
+        const string s = " 꿹ꓴ/ꥀ냘";
+        FontFamily fontFamily = SystemFonts.Get("Arial");
+        Font font = new(fontFamily, 10f);
+        _ = TextMeasurer.MeasureBounds(s, new TextOptions(font));
+    }
+}
+#endif