diff --git a/README.md b/README.md
index 350fac9418..be82acc678 100644
--- a/README.md
+++ b/README.md
@@ -177,7 +177,7 @@ bash scripts/download_data.sh
 
 The input images and target masks should be in the `data/imgs` and `data/masks` folders respectively (note that the `imgs` and `masks` folder should not contain any sub-folder or any other files, due to the greedy data-loader). For Carvana, images are RGB and masks are black and white.
 
-You can use your own dataset as long as you make sure it is loaded properly in `utils/data_loading.py`.
+You can use your own dataset as long as you make sure it is loaded properly in `utils/data_loading.py`. For example, loading RGB-coded black and white masks will not work with the data-loading code as-is.
 
 
 ---
diff --git a/utils/data_loading.py b/utils/data_loading.py
index 24893b02db..1e18817681 100644
--- a/utils/data_loading.py
+++ b/utils/data_loading.py
@@ -33,7 +33,11 @@ def preprocess(pil_img, scale, is_mask):
         pil_img = pil_img.resize((newW, newH), resample=Image.NEAREST if is_mask else Image.BICUBIC)
         img_ndarray = np.asarray(pil_img)
 
-        if not is_mask:
+        if is_mask:
+            if img_ndarray.ndim > 2:
+                # customize this function if you want it to support RGB masks
+                raise RuntimeError("Only black-and-white images are supported as masks.")
+        else:
             if img_ndarray.ndim == 2:
                 img_ndarray = img_ndarray[np.newaxis, ...]
             else: