diff --git a/src/blackmisc/stringutils.cpp b/src/blackmisc/stringutils.cpp
index 1f9a8379d..9eecf6ce3 100644
--- a/src/blackmisc/stringutils.cpp
+++ b/src/blackmisc/stringutils.cpp
@@ -26,28 +26,39 @@ namespace BlackMisc
         return splitString(s, [](QChar c) { return c == '\n' || c == '\r'; });
     }
 
-    QString boolToOnOff(bool v, bool i18n)
+    const QString &boolToOnOff(bool v)
     {
-        Q_UNUSED(i18n);
-        return v ? "on" : "off";
+        static const QString on("on");
+        static const QString off("off");
+        return v ? on : off;
     }
 
-    QString boolToYesNo(bool v, bool i18n)
+    const QString &boolToYesNo(bool v)
     {
-        Q_UNUSED(i18n);
-        return v ? "yes" : "no";
+        static const QString yes("yes");
+        static const QString no("no");
+        return v ? yes : no;
     }
 
-    QString boolToTrueFalse(bool v, bool i18n)
+    const QString &boolToTrueFalse(bool v)
     {
-        Q_UNUSED(i18n);
-        return v ? "true" : "false";
+        static const QString t("true");
+        static const QString f("false");
+        return v ? t : f;
     }
 
-    QString boolToEnabledDisabled(bool v, bool i18n)
+    const QString &boolToEnabledDisabled(bool v)
     {
-        Q_UNUSED(i18n);
-        return v ? "enabled" : "disabled";
+        static const QString e("enabled");
+        static const QString d("disabled");
+        return v ? e : d;
+    }
+
+    const QString &boolToNullNotNull(bool isNull)
+    {
+        static const QString n("null");
+        static const QString nn("not null");
+        return isNull ? n : nn;
     }
 
     bool stringToBool(const QString &string)
diff --git a/src/blackmisc/stringutils.h b/src/blackmisc/stringutils.h
index b2f87da76..34912706f 100644
--- a/src/blackmisc/stringutils.h
+++ b/src/blackmisc/stringutils.h
@@ -121,16 +121,19 @@ namespace BlackMisc
     }
 
     //! Bool to on/off
-    BLACKMISC_EXPORT QString boolToOnOff(bool v, bool  i18n = false);
+    BLACKMISC_EXPORT const QString &boolToOnOff(bool v);
 
     //! Bool to yes/no
-    BLACKMISC_EXPORT QString boolToYesNo(bool v, bool  i18n = false);
+    BLACKMISC_EXPORT const QString &boolToYesNo(bool v);
 
     //! Bool to true/false
-    BLACKMISC_EXPORT QString boolToTrueFalse(bool v, bool  i18n = false);
+    BLACKMISC_EXPORT const QString &boolToTrueFalse(bool v);
 
     //! Bool to enabled/disabled
-    BLACKMISC_EXPORT QString boolToEnabledDisabled(bool v, bool  i18n = false);
+    BLACKMISC_EXPORT const QString &boolToEnabledDisabled(bool v);
+
+    //! Bool isNull to null/no null
+    BLACKMISC_EXPORT const QString &boolToNullNotNull(bool isNull);
 
     //! Convert string to bool
     BLACKMISC_EXPORT bool stringToBool(const QString &boolString);
@@ -246,10 +249,10 @@ namespace BlackMisc
          * the derived class uses this macro to disambiguate the inherited members.
          */
 #       define BLACKMISC_DECLARE_USING_MIXIN_STRING(DERIVED)                \
-            using ::BlackMisc::Mixin::String<DERIVED>::toQString;           \
-            using ::BlackMisc::Mixin::String<DERIVED>::toFormattedQString;  \
-            using ::BlackMisc::Mixin::String<DERIVED>::toStdString;         \
-            using ::BlackMisc::Mixin::String<DERIVED>::stringForStreaming;
+    using ::BlackMisc::Mixin::String<DERIVED>::toQString;           \
+    using ::BlackMisc::Mixin::String<DERIVED>::toFormattedQString;  \
+    using ::BlackMisc::Mixin::String<DERIVED>::toStdString;         \
+    using ::BlackMisc::Mixin::String<DERIVED>::stringForStreaming;
     } // ns
 } // ns