diff --git a/ChatGPTExport/Assets/ExistingAssetLocator.cs b/ChatGPTExport/Assets/ExistingAssetLocator.cs
index 1d56f6f..53c8a2c 100644
--- a/ChatGPTExport/Assets/ExistingAssetLocator.cs
+++ b/ChatGPTExport/Assets/ExistingAssetLocator.cs
@@ -25,7 +25,17 @@ public void Add(string newFile)
 
         public string? GetMarkdownMediaAsset(AssetRequest assetRequest)
         {
-            // it may already exist in the destination directory from a previous export 
+            var invalidChars = fileSystem.Path.GetInvalidFileNameChars()
+                .Concat(new[] { '*', '?', fileSystem.Path.DirectorySeparatorChar, fileSystem.Path.AltDirectorySeparatorChar })
+                .Distinct()
+                .ToArray();
+
+            if (assetRequest.SearchPattern.IndexOfAny(invalidChars) >= 0)
+            {
+                return null;
+            }
+
+            // it may already exist in the destination directory from a previous export
             var destinationMatches = GetCachedDestinationFiles(assetRequest.SearchPattern).ToList();
             if (destinationMatches.Count == 0)
             {
diff --git a/ChatGTPExportTests/Assets/ExistingAssetLocatorTests.cs b/ChatGTPExportTests/Assets/ExistingAssetLocatorTests.cs
new file mode 100644
index 0000000..8eae3ee
--- /dev/null
+++ b/ChatGTPExportTests/Assets/ExistingAssetLocatorTests.cs
@@ -0,0 +1,47 @@
+using ChatGPTExport.Assets;
+using System.IO.Abstractions.TestingHelpers;
+
+namespace ChatGTPExportTests.Assets
+{
+    public class ExistingAssetLocatorTests
+    {
+        [Theory]
+        [InlineData("image*")]
+        [InlineData("image?")]
+        [InlineData("image/")]
+        [InlineData("image\\")]
+        [InlineData("invali<d")]
+        public void GetMarkdownMediaAsset_InvalidPattern_ReturnsNull(string pattern)
+        {
+            var fs = new MockFileSystem();
+            var destPath = fs.Path.Combine(fs.Path.DirectorySeparatorChar.ToString(), "dest");
+            fs.AddDirectory(destPath);
+            var destDir = fs.DirectoryInfo.New(destPath);
+            var locator = new ExistingAssetLocator(fs, destDir);
+
+            var request = new AssetRequest(pattern, "role", null, null);
+            var result = locator.GetMarkdownMediaAsset(request);
+
+            Assert.Null(result);
+        }
+
+        [Fact]
+        public void GetMarkdownMediaAsset_ValidPattern_ReturnsMarkdown()
+        {
+            var fs = new MockFileSystem();
+            var destPath = fs.Path.Combine(fs.Path.DirectorySeparatorChar.ToString(), "dest");
+            fs.AddDirectory(destPath);
+            var filePath = fs.Path.Combine(destPath, "image1.png");
+            fs.AddFile(filePath, new MockFileData("data"));
+
+            var destDir = fs.DirectoryInfo.New(destPath);
+            var locator = new ExistingAssetLocator(fs, destDir);
+
+            var request = new AssetRequest("image1", "role", null, null);
+            var result = locator.GetMarkdownMediaAsset(request);
+
+            Assert.NotNull(result);
+            Assert.Contains("image1.png", result);
+        }
+    }
+}