diff --git a/src/insets/InsetText.h b/src/insets/InsetText.h
index 35904398a8..3c242fd4f9 100644
--- a/src/insets/InsetText.h
+++ b/src/insets/InsetText.h
@@ -193,7 +193,7 @@ public:
 	/// returns the text to be used as tooltip
 	/// \param prefix: a string that will preced the tooltip,
 	/// e.g., "Index: ".
-	docstring toolTipText(docstring prefix = from_ascii("")) const;
+	docstring toolTipText(docstring prefix = empty_docstring()) const;
 
 	///
 	virtual docstring contextMenu(BufferView const & bv, int x, int y) const;
diff --git a/src/support/strfwd.h b/src/support/strfwd.h
index fcd97f5685..069357ff9c 100644
--- a/src/support/strfwd.h
+++ b/src/support/strfwd.h
@@ -67,8 +67,10 @@ typedef std::basic_ostream<char_type, std::char_traits<char_type> > odocstream;
 extern odocstream & operator<<(odocstream &, char);
 #endif
 
+// defined in lstrings.cpp
 docstring const & empty_docstring();
 std::string const & empty_string();
+// defined in docstring.cpp
 bool operator==(docstring const &, char const *);
 
 } // namespace lyx