diff --git a/mkvmuxerutil.cpp b/mkvmuxerutil.cpp
index 27ab15d..4dc0567 100644
--- a/mkvmuxerutil.cpp
+++ b/mkvmuxerutil.cpp
@@ -20,6 +20,7 @@
 #include <ctime>
 #include <new>
 
+#include "mkvmuxer.hpp"
 #include "mkvwriter.hpp"
 #include "webmids.hpp"
 
diff --git a/mkvmuxerutil.hpp b/mkvmuxerutil.hpp
index e318576..bf2214a 100644
--- a/mkvmuxerutil.hpp
+++ b/mkvmuxerutil.hpp
@@ -9,11 +9,11 @@
 #ifndef MKVMUXERUTIL_HPP
 #define MKVMUXERUTIL_HPP
 
-#include "mkvmuxer.hpp"
 #include "mkvmuxertypes.hpp"
 
 namespace mkvmuxer {
-
+class Cluster;
+class Frame;
 class IMkvWriter;
 
 const uint64 kEbmlUnknownValue = 0x01FFFFFFFFFFFFFFULL;