diff --git a/dlib/image_transforms/label_connected_blobs.h b/dlib/image_transforms/label_connected_blobs.h
index 4a77866fc..0fbade6a4 100644
--- a/dlib/image_transforms/label_connected_blobs.h
+++ b/dlib/image_transforms/label_connected_blobs.h
@@ -120,11 +120,21 @@ namespace dlib
         label_image_type& label_img
     )
     {
+        // make sure requires clause is not broken
+        DLIB_ASSERT(is_same_object(img, label_img) == false,
+            "\t unsigned long label_connected_blobs()"
+            << "\n\t The input image and output label image can't be the same object."
+            );
+
+
         std::stack<point> neighbors;
         label_img.set_size(img.nr(), img.nc());
         assign_all_pixels(label_img, 0);
         unsigned long next = 1;
 
+        if (img.size() == 0)
+            return 0;
+
         const rectangle area = get_rect(img);
 
         std::vector<point> window;
diff --git a/dlib/image_transforms/label_connected_blobs_abstract.h b/dlib/image_transforms/label_connected_blobs_abstract.h
index 75debd9a0..dac8252fa 100644
--- a/dlib/image_transforms/label_connected_blobs_abstract.h
+++ b/dlib/image_transforms/label_connected_blobs_abstract.h
@@ -153,6 +153,7 @@ namespace dlib
             - get_neighbors(point(c,r), neighbors) is a legal expression where neighbors 
               is of type std::vector<point>.
             - is_connected(img, point(c,r), point(c2,r2)) is a valid expression.
+            - is_same_object(img, label_img) == false
         ensures
             - This function labels each of the connected blobs in img with a unique integer 
               label.  
@@ -175,8 +176,11 @@ namespace dlib
                     - #label_img[r][c] == 0
                 - else
                     - #label_img[r][c] != 0
-            - returns max(array_to_matrix(label_img))+1
-              (i.e. returns a number one greater than the maximum blob id number)
+            - if (img.size() != 0) then 
+                - returns max(array_to_matrix(label_img))+1
+                  (i.e. returns a number one greater than the maximum blob id number)
+            - else
+                - returns 0
             - blob labels are contiguous, therefore, the number returned by this function is
               the number of blobs in the image (including the background blob).
             - It is guaranteed that is_connected() and is_background() will never be