diff --git a/src/support/docstring.C b/src/support/docstring.C
index ceffbc3832..7fd34ed9a9 100644
--- a/src/support/docstring.C
+++ b/src/support/docstring.C
@@ -122,7 +122,7 @@ lyx::docstring operator+(char l, lyx::docstring const & r)
 }
 
 
-lyx::docstring operator+=(lyx::docstring & l, char const * r)
+lyx::docstring & operator+=(lyx::docstring & l, char const * r)
 {
 	for (char const * c = r; *c; ++c) {
 		BOOST_ASSERT(static_cast<unsigned char>(*c) < 0x80);
@@ -132,7 +132,7 @@ lyx::docstring operator+=(lyx::docstring & l, char const * r)
 }
 
 
-lyx::docstring operator+=(lyx::docstring & l, char r)
+lyx::docstring & operator+=(lyx::docstring & l, char r)
 {
 	BOOST_ASSERT(static_cast<unsigned char>(r) < 0x80);
 	l.push_back(r);
diff --git a/src/support/docstring.h b/src/support/docstring.h
index 1cff5109a0..072e76155b 100644
--- a/src/support/docstring.h
+++ b/src/support/docstring.h
@@ -64,10 +64,10 @@ lyx::docstring operator+(lyx::docstring const & l, char r);
 lyx::docstring operator+(char l, lyx::docstring const & r);
 
 /// Append a C string of ASCII characters to a docstring
-lyx::docstring operator+=(lyx::docstring &, char const *);
+lyx::docstring & operator+=(lyx::docstring &, char const *);
 
 /// Append a single ASCII character to a docstring
-lyx::docstring operator+=(lyx::docstring & l, char r);
+lyx::docstring & operator+=(lyx::docstring & l, char r);
 
 
 #if SIZEOF_WCHAR_T != 4 && defined(__GNUC__) && defined(__GNUC_MINOR__) && __GNUC__ == 3 && __GNUC_MINOR__ < 4