diff --git a/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileSystem.cs b/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileSystem.cs
index 8b1b620a2..3f71206cb 100644
--- a/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileSystem.cs
+++ b/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileSystem.cs
@@ -156,7 +156,12 @@ private string GetPathWithCorrectDirectoryCapitalization(string fullPath)
 
             if (DirectoryExistsWithoutFixingPath(leftHalf))
             {
-                string baseDirectory = files[leftHalf].Path;
+                string baseDirectory;
+                lock (files)
+                {
+                    baseDirectory = files[leftHalf].Path;
+                }
+
                 return baseDirectory + Path.DirectorySeparatorChar + rightHalf;
             }
         }
@@ -581,4 +586,4 @@ private class FileSystemEntry
         public string Path { get; set; }
         public MockFileData Data { get; set; }
     }
-}
\ No newline at end of file
+}