diff --git a/src/frontends/qt/GuiCitation.cpp b/src/frontends/qt/GuiCitation.cpp
index e9f9ec9937..1800543d4a 100644
--- a/src/frontends/qt/GuiCitation.cpp
+++ b/src/frontends/qt/GuiCitation.cpp
@@ -835,7 +835,7 @@ void GuiCitation::init()
 
 void GuiCitation::findKey(BiblioInfo const & bi,
 	QString const & str, bool only_keys,
-	docstring field, docstring entry_type,
+	docstring const & field, docstring const & entry_type,
 	bool case_sensitive, bool reg_exp, bool reset)
 {
 	// FIXME THREAD
diff --git a/src/frontends/qt/GuiCitation.h b/src/frontends/qt/GuiCitation.h
index 948933a8e3..3fecfb208f 100644
--- a/src/frontends/qt/GuiCitation.h
+++ b/src/frontends/qt/GuiCitation.h
@@ -129,8 +129,8 @@ private:
 		BiblioInfo const & bi, //< optimize by passing this
 		QString const & str, //< string expression
 		bool only_keys, //< set to true if only keys shall be searched.
-		docstring field, //<field to search, empty for all fields
-		docstring entryType, //<entry type to display, empty for all
+		docstring const & field, //<field to search, empty for all fields
+		docstring const & entryType, //<entry type to display, empty for all
 		bool case_sensitive, //< set to true for case sensitive search.
 		bool reg_exp, //< set to true if \c str is a regular expression.
 		bool reset = false //< whether to reset and search all keys