diff --git a/crates/engine/src/brush_cmds.rs b/crates/engine/src/brush_cmds.rs
index e7178a74..1fa109e8 100644
--- a/crates/engine/src/brush_cmds.rs
+++ b/crates/engine/src/brush_cmds.rs
@@ -167,6 +167,12 @@ fn stroke_with(s: &mut Session, p: &Value, label: &str, brush: BrushSettings, pt
         let sel = doc.selection.clone();
         let (surf, lock) = crate::channel_cmds::target_surface(doc, id, p)?;
         let mut brush = brush;
+        // Erasing a layer with locked transparency (e.g. the Background) can't remove opacity, so
+        // it paints the background colour instead (Photoshop). `gray` targets are handled above.
+        if brush.erase && lock {
+            brush.erase = false;
+            brush.color = bg;
+        }
         if auto_erase {
             // Pencil Auto Erase: starting on foreground-coloured pixels paints the background colour.
             let c = surf.rgba(pts[0].x.floor() as i32, pts[0].y.floor() as i32);
diff --git a/crates/engine/src/brush_cmds/tests.rs b/crates/engine/src/brush_cmds/tests.rs
index f60eaa91..c09d1f8f 100644
--- a/crates/engine/src/brush_cmds/tests.rs
+++ b/crates/engine/src/brush_cmds/tests.rs
@@ -226,3 +226,19 @@ fn brush_blend_mode_multiply() {
     let n = rgba(&s2, 20, 20);
     assert!(n[2] > 0.9 && n[0] < 0.1, "normal paints blue: {n:?}");
 }
+
+#[test]
+fn eraser_on_background_layer_paints_the_background_colour() {
+    // Issue #15: erasing the Background layer (locked transparency) must paint the background
+    // colour, not silently do nothing (opacity can't be removed there).
+    let mut s = Session::new();
+    s.execute("file.new", json!({"width": 20, "height": 20, "background": "white"})).unwrap();
+    s.execute("tools.setColors", json!({"background": "#ff0000"})).unwrap();
+    s.execute("paint.stroke", json!({"points": [[2, 10], [18, 10]], "size": 8, "erase": true, "brush": {"hardness": 1.0}})).unwrap();
+    let st = s.active().unwrap();
+    let c = st.doc.layers[0].surface().unwrap().rgba(10, 10);
+    assert!(c[0] > 0.9 && c[1] < 0.1 && c[2] < 0.1 && c[3] > 0.99, "erased to opaque background red: {c:?}");
+    // A corner the stroke didn't touch stays white.
+    let w = st.doc.layers[0].surface().unwrap().rgba(10, 2);
+    assert!(w[0] > 0.9 && w[1] > 0.9 && w[2] > 0.9, "untouched stays white: {w:?}");
+}