diff --git a/README.md b/README.md
index 4d786d2..1cbbd5b 100644
--- a/README.md
+++ b/README.md
@@ -41,6 +41,11 @@ Actions:
 * `-`: decrease scale (dezoom)
 * `=`: reset scale
 
+# Mouse control
+* Right click: reset selection.
+* Hold left click: add to selection.
+* Hold left click + shift: select rectangle.
+
 # License
 Copyright (c) 2021 KikooDX
 
diff --git a/src/level.zig b/src/level.zig
index 4b22064..d52953c 100644
--- a/src/level.zig
+++ b/src/level.zig
@@ -104,8 +104,11 @@ pub fn draw_selection(self: *Self, scale: u16, offset: Vec2) void {
 /// Set the `state` of a cell at `cursor` if possible
 pub fn select_cell(self: *Self, cursor: Vec2, state: bool) void {
     const target: u32 = @intCast(u32, cursor.y) * self.width + @intCast(u32, cursor.x);
-    if (target < self.width * self.height and target >= 0)
+    if (cursor.x < self.width and cursor.y < self.height and
+        target < self.width * self.height and target >= 0)
+    {
         self.selection[target] = state;
+    }
 }
 
 /// Reset state of selection: `state` everywhere.