diff --git a/include/wx/ffile.h b/include/wx/ffile.h
index e9eaf36db7..8f8ffde014 100644
--- a/include/wx/ffile.h
+++ b/include/wx/ffile.h
@@ -86,7 +86,8 @@ public:
     // get current file length
   size_t Length() const;
 
-  // simple accessors
+  // simple accessors: note that Eof() and Error() may only be called if
+  // IsOpened()!
     // is file opened?
   bool IsOpened() const { return m_fp != NULL; }
     // is end of file reached?
diff --git a/src/common/ffile.cpp b/src/common/ffile.cpp
index 95105fbfed..503e5bfa0d 100644
--- a/src/common/ffile.cpp
+++ b/src/common/ffile.cpp
@@ -206,6 +206,9 @@ bool wxFFile::Seek(long ofs, wxSeekMode mode)
 
 size_t wxFFile::Tell() const
 {
+    wxCHECK_MSG( IsOpened(), (size_t)-1,
+                 _T("wxFFile::Tell(): file is closed!") );
+
     long rc = ftell(m_fp);
     if ( rc == -1 )
     {
@@ -218,6 +221,9 @@ size_t wxFFile::Tell() const
 
 size_t wxFFile::Length() const
 {
+    wxCHECK_MSG( IsOpened(), (size_t)-1,
+                 _T("wxFFile::Length(): file is closed!") );
+
     wxFFile& self = *(wxFFile *)this;   // const_cast
 
     size_t posOld = Tell();