diff --git a/src/settings_gui.cpp b/src/settings_gui.cpp
index e7d1f102f9..c9c2d81aa5 100644
--- a/src/settings_gui.cpp
+++ b/src/settings_gui.cpp
@@ -156,23 +156,21 @@ static void AddCustomRefreshRates()
 static const int SCALE_NMARKS = (MAX_INTERFACE_SCALE - MIN_INTERFACE_SCALE) / 25 + 1; // Show marks at 25% increments
 static const int VOLUME_NMARKS = 9; // Show 5 values and 4 empty marks.
 
-static StringID ScaleMarkFunc(int, int, int value)
+static std::optional<std::string> ScaleMarkFunc(int, int, int value)
 {
 	/* Label only every 100% mark. */
-	if (value % 100 != 0) return STR_NULL;
+	if (value % 100 != 0) return std::string{};
 
-	SetDParam(0, value / 100);
-	SetDParam(1, 0);
-	return STR_GAME_OPTIONS_GUI_SCALE_MARK;
+	return GetString(STR_GAME_OPTIONS_GUI_SCALE_MARK, value / 100, 0);
 }
 
-static StringID VolumeMarkFunc(int, int mark, int value)
+static std::optional<std::string> VolumeMarkFunc(int, int mark, int value)
 {
 	/* Label only every other mark. */
-	if (mark % 2 != 0) return STR_NULL;
+	if (mark % 2 != 0) return std::string{};
 
-	SetDParam(0, value / 31 * 25); // 0-127 does not map nicely to 0-100. Dividing first gives us nice round numbers.
-	return STR_GAME_OPTIONS_VOLUME_MARK;
+	// 0-127 does not map nicely to 0-100. Dividing first gives us nice round numbers.
+	return GetString(STR_GAME_OPTIONS_VOLUME_MARK, value / 31 * 25);
 }
 
 static constexpr NWidgetPart _nested_social_plugins_widgets[] = {
diff --git a/src/slider.cpp b/src/slider.cpp
index 69194c402c..362174ee0e 100644
--- a/src/slider.cpp
+++ b/src/slider.cpp
@@ -56,18 +56,18 @@ void DrawSliderWidget(Rect r, int min_value, int max_value, int nmarks, int valu
 		for (int mark = 0; mark < nmarks; ++mark) {
 			const int mark_value = (max_value * mark) / (nmarks - 1);
 
-			const StringID str = mark_func(nmarks, mark, mark_value + min_value);
-			if (str == INVALID_STRING_ID) continue;
+			auto str = mark_func(nmarks, mark, mark_value + min_value);
+			if (!str.has_value()) continue;
 
 			x = mark_value;
 			if (_current_text_dir == TD_RTL) x = max_value - mark_value;
 			x = r.left + (x * (r.right - r.left - sw) / max_value) + sw / 2;
-			GfxDrawLine(x, r.bottom - ha + 1, x, r.bottom + (str == STR_NULL ? 0 : WidgetDimensions::scaled.hsep_normal), shadow, t);
-			if (str == STR_NULL) continue;
+			GfxDrawLine(x, r.bottom - ha + 1, x, r.bottom + (str->empty() ? 0 : WidgetDimensions::scaled.hsep_normal), shadow, t);
+			if (str->empty()) continue;
 
-			Dimension d = GetStringBoundingBox(str, FS_SMALL);
+			Dimension d = GetStringBoundingBox(*str, FS_SMALL);
 			x = Clamp(x - d.width / 2, r.left, r.right - d.width);
-			DrawString(x, x + d.width, r.bottom + 1 + WidgetDimensions::scaled.hsep_normal, str, TC_BLACK, SA_CENTER, false, FS_SMALL);
+			DrawString(x, x + d.width, r.bottom + 1 + WidgetDimensions::scaled.hsep_normal, *str, TC_BLACK, SA_CENTER, false, FS_SMALL);
 		}
 	}
 
diff --git a/src/slider_func.h b/src/slider_func.h
index cb5b97fe06..c4938644db 100644
--- a/src/slider_func.h
+++ b/src/slider_func.h
@@ -13,7 +13,7 @@
 #include "core/geometry_type.hpp"
 #include "strings_type.h"
 
-using SliderMarkFunc = StringID(int nmarks, int mark, int value);
+using SliderMarkFunc = std::optional<std::string>(int nmarks, int mark, int value);
 void DrawSliderWidget(Rect r, int min_value, int max_value, int nmarks, int value, SliderMarkFunc *mark_func);
 bool ClickSliderWidget(Rect r, Point pt, int min_value, int max_value, int nmarks, int &value);