diff --git a/rtengine/boxblur.h b/rtengine/boxblur.h
index 805575b77..71452ceae 100644
--- a/rtengine/boxblur.h
+++ b/rtengine/boxblur.h
@@ -35,6 +35,8 @@ namespace rtengine
 template<class T, class A> void boxblur (T** src, A** dst, int radx, int rady, int W, int H)
 {
     //box blur image; box range = (radx,rady)
+    assert(2*radx+1 < W);
+    assert(2*rady+1 < H);
 
     AlignedBuffer<float>* buffer = new AlignedBuffer<float> (W * H);
     float* temp = buffer->data;
diff --git a/rtengine/guidedfilter.cc b/rtengine/guidedfilter.cc
index 3000e1d5d..f6b702a73 100644
--- a/rtengine/guidedfilter.cc
+++ b/rtengine/guidedfilter.cc
@@ -110,7 +110,7 @@ void guidedFilter(const array2D<float> &guide, const array2D<float> &src, array2
     const auto f_mean =
         [](array2D<float> &d, array2D<float> &s, int rad) -> void
         {
-            rad = min(rad, s.width() / 2, s.height() / 2);
+            rad = LIM(rad, 0, (min(s.width(), s.height()) - 1) / 2 - 1);
             boxblur<float, float>(s, d, rad, rad, s.width(), s.height());
         };