diff --git a/Source/mmap.h b/Source/mmap.h
index c1c6627c..9f144498 100644
--- a/Source/mmap.h
+++ b/Source/mmap.h
@@ -72,7 +72,7 @@ class MMapFile : public IMMap
 #else
     FILE *m_hFile;
     int m_hFileDesc;
-    int m_iMappedSize;
+    mutable int m_iMappedSize;
 #endif
     mutable void *m_pView;
     mutable int m_iSize;