diff --git a/simgear/io/sg_mmap.cxx b/simgear/io/sg_mmap.cxx
index 119a2369..c3309512 100755
--- a/simgear/io/sg_mmap.cxx
+++ b/simgear/io/sg_mmap.cxx
@@ -71,6 +71,7 @@ public:
 };
 
 SGMMapFile::SGMMapFile( )
+    : d(new SGMMapFilePrivate)
 {
     set_type( sgFileType );
 }