diff --git a/src/MarkdownSnippets/Processing/SnippetKey.cs b/src/MarkdownSnippets/Processing/SnippetKey.cs
index 252b4ae1..476107b7 100644
--- a/src/MarkdownSnippets/Processing/SnippetKey.cs
+++ b/src/MarkdownSnippets/Processing/SnippetKey.cs
@@ -11,8 +11,12 @@ public static bool ExtractStartCommentSnippet(Line line, [NotNullWhen(true)] out
 
         var substring = lineCurrent[14..];
         var indexOf = substring.IndexOf("-->");
-        key = substring[..indexOf]
-            .Trim();
+        if (indexOf < 0)
+        {
+            throw new SnippetException($"Could not find closing '-->' in: {line.Original}. Path: {line.Path}. Line: {line.LineNumber}");
+        }
+
+        key = substring[..indexOf].Trim();
         return true;
     }
 
@@ -32,6 +36,11 @@ public static bool ExtractStartCommentWebSnippet(Line line, [NotNullWhen(true)]
 
         var substring = lineCurrent[18..]; // after "<!-- web-snippet: "
         var indexOf = substring.IndexOf("-->");
+        if (indexOf < 0)
+        {
+            throw new SnippetException($"Could not find closing '-->' in: {line.Original}. Path: {line.Path}. Line: {line.LineNumber}");
+        }
+
         var value = substring[..indexOf].Trim();
 
         // Check for optional second URL separated by whitespace
diff --git a/src/Tests/MarkdownProcessor/SnippetKey_ExtractStartCommentSnippet.cs b/src/Tests/MarkdownProcessor/SnippetKey_ExtractStartCommentSnippet.cs
index 0873c112..6ca54769 100644
--- a/src/Tests/MarkdownProcessor/SnippetKey_ExtractStartCommentSnippet.cs
+++ b/src/Tests/MarkdownProcessor/SnippetKey_ExtractStartCommentSnippet.cs
@@ -13,4 +13,13 @@ public void Simple()
         Assert.True(SnippetKey.ExtractStartCommentSnippet(new("<!-- snippet: snippet -->", "path", 1), out var key));
         Assert.Equal("snippet", key);
     }
+
+    [Fact]
+    public void MissingClosingComment_Throws()
+    {
+        var line = new Line("<!-- snippet: my-snippet", "test.md", 5);
+        var exception = Assert.Throws<SnippetException>(() => SnippetKey.ExtractStartCommentSnippet(line, out _));
+        Assert.Contains("-->", exception.Message);
+        Assert.Contains("test.md", exception.Message);
+    }
 }
\ No newline at end of file
diff --git a/src/Tests/MarkdownProcessor/SnippetKey_ExtractStartCommentWebSnippet.cs b/src/Tests/MarkdownProcessor/SnippetKey_ExtractStartCommentWebSnippet.cs
new file mode 100644
index 00000000..f13da0b9
--- /dev/null
+++ b/src/Tests/MarkdownProcessor/SnippetKey_ExtractStartCommentWebSnippet.cs
@@ -0,0 +1,19 @@
+public class SnippetKey_ExtractStartCommentWebSnippet
+{
+    [Fact]
+    public void Simple()
+    {
+        Assert.True(SnippetKey.ExtractStartCommentWebSnippet(new("<!-- web-snippet: https://example.com/file.cs#mysnippet -->", "path", 1), out var url, out var key));
+        Assert.Equal("https://example.com/file.cs", url);
+        Assert.Equal("mysnippet", key);
+    }
+
+    [Fact]
+    public void MissingClosingComment_Throws()
+    {
+        var line = new Line("<!-- web-snippet: https://example.com/file.cs#mysnippet", "test.md", 10);
+        var exception = Assert.Throws<SnippetException>(() => SnippetKey.ExtractStartCommentWebSnippet(line, out _, out _));
+        Assert.Contains("-->", exception.Message);
+        Assert.Contains("test.md", exception.Message);
+    }
+}
\ No newline at end of file