diff --git a/src/surface_normal_impl.hpp b/src/surface_normal_impl.hpp
index 2f2899a..dc6f24f 100644
--- a/src/surface_normal_impl.hpp
+++ b/src/surface_normal_impl.hpp
@@ -38,7 +38,7 @@ depth_to_normals_rgb_inner(const ImageView<const T> &depth, ImageView<uint8_t, 3
       int x = center_col + j;
       int y = center_row + i;
 
-      if (x >= depth.width || y >= depth.height) {
+      if (x < 0 || x >= depth.width || y < 0 || y >= depth.height) {
         continue;
       }
 
@@ -81,4 +81,4 @@ depth_to_normals_rgb_inner(const ImageView<const T> &depth, ImageView<uint8_t, 3
   normals.at(center_row, center_col, 1) = f2b(normal(2));
   normals.at(center_row, center_col, 2) = f2b(normal(1));
 }
-} // namespace surface_normal
\ No newline at end of file
+} // namespace surface_normal