diff --git a/backend/processing/registry.py b/backend/processing/registry.py
index 88bda6f..a63397e 100644
--- a/backend/processing/registry.py
+++ b/backend/processing/registry.py
@@ -327,6 +327,7 @@ def gradient_abs_sum(image, params):
     Use it to emphasize prominent edges before combining them with a sharpened image.
     """
 
+    grayscale_input = image.ndim == 2
     operator = params.get("operator", "sobel")
     gray = to_gray(image).astype(np.float32)
     if operator == "roberts":
@@ -337,7 +338,8 @@ def gradient_abs_sum(image, params):
         gy = np.array([[-1, 0, 1], [-2, 0, 2], [-1, 0, 1]], dtype=np.float32)
     fx = cv2.filter2D(gray, cv2.CV_32F, gx, borderType=cv2.BORDER_REFLECT)
     fy = cv2.filter2D(gray, cv2.CV_32F, gy, borderType=cv2.BORDER_REFLECT)
-    return gray_to_rgb(normalize_to_uint8(np.abs(fx) + np.abs(fy)))
+    gradient = normalize_to_uint8(np.abs(fx) + np.abs(fy))
+    return gradient if grayscale_input else gray_to_rgb(gradient)
 
 
 def rgb_channel(image, params):
diff --git a/backend/processing/tests/test_api.py b/backend/processing/tests/test_api.py
index 0dae9b4..429aed5 100644
--- a/backend/processing/tests/test_api.py
+++ b/backend/processing/tests/test_api.py
@@ -385,6 +385,40 @@ class ApiTests(TestCase):
         self.assertEqual(noisy.data["channels"], 1)
         self.assertEqual(noisy.data["color_mode"], "L")
 
+    def test_gradient_mask_from_grayscale_can_combine_with_grayscale_state(self):
+        upload = self.client.post("/api/images/", {"image": png_upload()}, format="multipart")
+        s0_id = upload.data["states"][0]["state_id"]
+        gray = self.client.post(
+            f"/api/states/{s0_id}/operations/",
+            {"operation": "rgb_to_gray", "params": {"red_weight": 0.299, "green_weight": 0.587, "blue_weight": 0.114}},
+            format="json",
+        )
+        gradient = self.client.post(
+            f"/api/states/{gray.data['state_id']}/operations/",
+            {"operation": "gradient_abs_sum", "params": {"operator": "sobel"}},
+            format="json",
+        )
+
+        self.assertEqual(gradient.status_code, 201)
+        self.assertEqual(gradient.data["channels"], 1)
+        self.assertEqual(gradient.data["color_mode"], "L")
+
+        added = self.client.post(
+            "/api/states/combine/",
+            {"operation": "add", "state_ids": [gray.data["state_id"], gradient.data["state_id"]]},
+            format="json",
+        )
+        subtracted = self.client.post(
+            "/api/states/combine/",
+            {"operation": "subtract", "state_ids": [gray.data["state_id"], gradient.data["state_id"]]},
+            format="json",
+        )
+
+        self.assertEqual(added.status_code, 201)
+        self.assertEqual(added.data["channels"], 1)
+        self.assertEqual(subtracted.status_code, 201)
+        self.assertEqual(subtracted.data["channels"], 1)
+
     @patch("processing.services.run_batch_job.delay")
     def test_batch_returns_job_id(self, delay):
         first = self.client.post("/api/images/", {"image": png_upload(name="a.png")}, format="multipart")