diff --git a/src/heightmap.cpp b/src/heightmap.cpp
index 34300fd18d..6cee98407b 100644
--- a/src/heightmap.cpp
+++ b/src/heightmap.cpp
@@ -474,8 +474,8 @@ void FixSlopes()
 		}
 	}
 
-	extern bool CheckMapEdgesAreWater();
-	if (_settings_game.construction.map_edge_mode != 0 && !CheckMapEdgesAreWater()) {
+	extern bool CheckMapEdgesAreWater(bool allow_non_flat_void);
+	if (_settings_game.construction.map_edge_mode != 0 && !CheckMapEdgesAreWater(false)) {
 		_settings_game.construction.map_edge_mode = 0;
 	}
 }
diff --git a/src/settings.cpp b/src/settings.cpp
index 4102388d01..4d8155a890 100644
--- a/src/settings.cpp
+++ b/src/settings.cpp
@@ -1681,18 +1681,27 @@ static void UpdateFreeformEdges(int32_t new_value)
 	MarkWholeScreenDirty();
 }
 
-bool CheckMapEdgesAreWater()
+bool CheckMapEdgesAreWater(bool allow_non_flat_void)
 {
-	auto check_tile = [&](uint x, uint y) -> bool {
+	auto check_tile = [&](uint x, uint y, Slope inner_edge) -> bool {
 		int h = 0;
 		Slope slope = GetTilePixelSlopeOutsideMap(x, y, &h);
-		return slope == SLOPE_FLAT && h == 0;
+		if (slope == SLOPE_FLAT && h == 0) return true;
+		if (allow_non_flat_void && h == 0 && (slope & inner_edge) == 0 && IsTileType(TileXY(x, y), MP_VOID)) return true;
+		return false;
 	};
-	for (uint x = 0; x <= MapMaxX(); x++) {
-		if (!check_tile(x, 0) || !check_tile(x, MapMaxY())) return false;
+	check_tile(        0,         0, SLOPE_S);
+	check_tile(        0, MapMaxY(), SLOPE_W);
+	check_tile(MapMaxX(),         0, SLOPE_E);
+	check_tile(MapMaxX(), MapMaxY(), SLOPE_N);
+
+	for (uint x = 1; x < MapMaxX(); x++) {
+		if (!check_tile(x, 0, SLOPE_SE)) return false;
+		if (!check_tile(x, MapMaxY(), SLOPE_NW)) return false;
 	}
 	for (uint y = 1; y < MapMaxY(); y++) {
-		if (!check_tile(0, y) || !check_tile(MapMaxX(), y)) return false;
+		if (!check_tile(0, y, SLOPE_SW)) return false;
+		if (!check_tile(MapMaxX(), y, SLOPE_NE)) return false;
 	}
 
 	return true;
@@ -1702,7 +1711,7 @@ static bool CheckMapEdgeMode(int32_t &new_value)
 {
 	if (_game_mode == GM_MENU || !_settings_game.construction.freeform_edges || new_value == 0) return true;
 
-	if (!CheckMapEdgesAreWater()) {
+	if (!CheckMapEdgesAreWater(true)) {
 		ShowErrorMessage(STR_CONFIG_SETTING_EDGES_NOT_WATER, INVALID_STRING_ID, WL_ERROR);
 		return false;
 	}
@@ -1710,6 +1719,22 @@ static bool CheckMapEdgeMode(int32_t &new_value)
 	return true;
 }
 
+static void MapEdgeModeChanged(int32_t new_value)
+{
+	MarkAllViewportsDirty(new_value);
+
+	if (_game_mode == GM_MENU || !_settings_game.construction.freeform_edges || new_value == 0) return;
+
+	for (uint x = 0; x <= MapMaxX(); x++) {
+		SetTileHeight(TileXY(x, 0), 0);
+		SetTileHeight(TileXY(x, MapMaxY()), 0);
+	}
+	for (uint y = 1; y < MapMaxY(); y++) {
+		SetTileHeight(TileXY(0, y), 0);
+		SetTileHeight(TileXY(MapMaxX(), y), 0);
+	}
+}
+
 /**
  * Changing the setting "allow multiple NewGRF sets" is not allowed
  * if there are vehicles.
diff --git a/src/table/settings/world_settings.ini b/src/table/settings/world_settings.ini
index 2cf5a72e5f..b10b4ed04e 100644
--- a/src/table/settings/world_settings.ini
+++ b/src/table/settings/world_settings.ini
@@ -14,6 +14,7 @@ static bool CheckMaxHeightLevel(int32_t &new_value);
 static bool CheckFreeformEdges(int32_t &new_value);
 static void UpdateFreeformEdges(int32_t new_value);
 static bool CheckMapEdgeMode(int32_t &new_value);
+static void MapEdgeModeChanged(int32_t new_value);
 static void ClimateThresholdModeChanged(int32_t new_value);
 static void PublicRoadsSettingChange(int32_t new_value);
 static void MarkAllViewportsDirty(int32_t new_value);
@@ -944,7 +945,7 @@ str      = STR_CONFIG_SETTING_MAP_EDGE_MODE
 strhelp  = STR_CONFIG_SETTING_MAP_EDGE_MODE_HELPTEXT
 strval   = STR_CONFIG_SETTING_MAP_EDGE_MODE_DEFAULT
 pre_cb   = CheckMapEdgeMode
-post_cb  = MarkAllViewportsDirty
+post_cb  = MapEdgeModeChanged
 cat      = SC_ADVANCED
 
 [SDT_VAR]