diff --git a/Frontmatter.Test/UnclosedOpeningRuleTests.cs b/Frontmatter.Test/UnclosedOpeningRuleTests.cs
new file mode 100644
index 0000000..3d41546
--- /dev/null
+++ b/Frontmatter.Test/UnclosedOpeningRuleTests.cs
@@ -0,0 +1,49 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Tests that a document opening with a <c>---</c> rule that never closes is treated as having no frontmatter.
+/// </summary>
+[TestClass]
+public class UnclosedOpeningRuleTests
+{
+	private static readonly string Nl = Environment.NewLine;
+
+	private static readonly string Document = $"---{Nl}{Nl}# Heading{Nl}{Nl}Text{Nl}";
+
+	[TestMethod]
+	public void HasFrontmatter_OpeningRuleNeverCloses_IsFalse()
+	{
+		Assert.IsFalse(Frontmatter.HasFrontmatter($"---{Nl}{Nl}# Heading{Nl}"));
+		Assert.IsFalse(Frontmatter.HasFrontmatter(Document));
+	}
+
+	[TestMethod]
+	public void ExtractFrontmatter_OpeningRuleNeverCloses_IsNull()
+	{
+		Assert.IsNull(Frontmatter.ExtractFrontmatter(Document));
+	}
+
+	[TestMethod]
+	public void AddFrontmatter_OpeningRuleNeverCloses_WritesAHeaderInFrontOfTheRule()
+	{
+		string result = Frontmatter.AddFrontmatter(Document, new Dictionary<string, object> { ["title"] = "T" });
+
+		Assert.AreEqual($"---{Nl}title: T{Nl}---{Nl}---{Nl}{Nl}# Heading{Nl}{Nl}Text{Nl}", result);
+
+		Dictionary<string, object>? frontmatter = Frontmatter.ExtractFrontmatter(result);
+		Assert.IsNotNull(frontmatter);
+		Assert.HasCount(1, frontmatter);
+		Assert.AreEqual("T", frontmatter["title"]);
+		Assert.AreEqual($"---{Nl}{Nl}# Heading{Nl}{Nl}Text", Frontmatter.ExtractBody(result));
+	}
+
+	[TestMethod]
+	public void RemoveFrontmatter_OpeningRuleNeverCloses_LeavesTheDocumentAlone()
+	{
+		Assert.AreEqual(Document, Frontmatter.RemoveFrontmatter(Document));
+	}
+}
diff --git a/Frontmatter/Frontmatter.cs b/Frontmatter/Frontmatter.cs
index 76b1f83..f0b715f 100644
--- a/Frontmatter/Frontmatter.cs
+++ b/Frontmatter/Frontmatter.cs
@@ -144,12 +144,28 @@ public static string CombineFrontmatter(string input, FrontmatterNaming property
 	/// Checks if a markdown document contains frontmatter.
 	/// </summary>
 	/// <param name="input">The markdown document content as a string.</param>
+	/// <remarks>
+	/// Only a closed block counts. A document that opens with a <c>---</c> line that never closes, such as
+	/// a markdown thematic break, has no frontmatter, so <see cref="AddFrontmatter"/> writes a header in front
+	/// of it instead of leaving it alone.
+	/// </remarks>
 	/// <returns>True if the document contains frontmatter, false otherwise.</returns>
 	/// <exception cref="ArgumentNullException">Thrown when input is null.</exception>
 	public static bool HasFrontmatter(string input)
 	{
 		Ensure.NotNull(input);
 
+		return TrySplitFrontmatterBlocks(input, out _, out _);
+	}
+
+	/// <summary>
+	/// Checks whether the first line of a document is a frontmatter delimiter, whether or not a closing
+	/// delimiter follows it.
+	/// </summary>
+	/// <param name="input">The document to inspect.</param>
+	/// <returns>True if the document's first line is a delimiter, false otherwise.</returns>
+	private static bool OpensWithDelimiter(string input)
+	{
 		// The opening delimiter follows the same rule as the closing one, so trailing whitespace an editor
 		// left behind does not hide the header. A leading byte order mark is skipped, since callers that
 		// decode bytes or streams themselves keep it where File.ReadAllText would strip it.
@@ -311,8 +327,9 @@ private static List<Dictionary<string, object>> ExtractFrontmatterObjects(string
 	/// <param name="input">The markdown document content as a string.</param>
 	/// <param name="body">Output parameter that will contain the markdown body without frontmatter.</param>
 	/// <param name="hasUnreadableBlock">
-	/// True when the document opens with a frontmatter delimiter but a non-blank block failed to parse, or no
-	/// closed block could be found. Such text is missing from the returned objects but not from the document.
+	/// True when a non-blank frontmatter block failed to parse. Such text is missing from the returned objects
+	/// but not from the document. An opening delimiter that never closes is not frontmatter, so it is not
+	/// reported here.
 	/// </param>
 	/// <returns>A collection of dictionaries representing each frontmatter section that parsed.</returns>
 	private static List<Dictionary<string, object>> ExtractFrontmatterObjects(string input, out string body, out bool hasUnreadableBlock)
@@ -322,7 +339,6 @@ private static List<Dictionary<string, object>> ExtractFrontmatterObjects(string
 
 		if (!TrySplitFrontmatterBlocks(input, out List<string> blocks, out body))
 		{
-			hasUnreadableBlock = HasFrontmatter(input);
 			return frontmatterObjects;
 		}
 
@@ -385,7 +401,7 @@ private static bool TrySplitFrontmatterBlocks(string input, out List<string> blo
 		blocks = [];
 		body = input;
 
-		if (!HasFrontmatter(input))
+		if (!OpensWithDelimiter(input))
 		{
 			return false;
 		}