diff --git a/rtengine/camconst.json b/rtengine/camconst.json
index 679c7a134..7330ff27c 100644
--- a/rtengine/camconst.json
+++ b/rtengine/camconst.json
@@ -1933,6 +1933,11 @@ Camera constants:
         "raw_crop": [ 12, 0, -110, -62 ] // for small size all numbers/2
     },
 
+    { // Quality C, only raw crop for now - see #3858
+        "make_model": "Sigma sd Quattro",
+        "raw_crop": [ 200, 74, 5632, 3698 ]
+    },
+
     { // Quality A, correction for color matrix from Colin Walker's d50 to dng d65
         "make_model": "Sony NEX-C3",
         //"dcraw_matrix": [ 5130,-1055,-269,-4473,11797,3050,-701,1310,7121 ], // Colin walker's d50 kept for possible consistency issues
diff --git a/rtengine/rawimage.cc b/rtengine/rawimage.cc
index c0fd1f576..d8dc7eb1d 100644
--- a/rtengine/rawimage.cc
+++ b/rtengine/rawimage.cc
@@ -547,8 +547,8 @@ int RawImage::loadRaw (bool loadData, unsigned int imageNum, bool closeFile, Pro
             crop_masked_pixels();
             free (raw_image);
             raw_image = nullptr;
-        } else {
-            if (is_foveon && cc && cc->has_rawCrop()) { // foveon images
+        } else { 
+            if (get_maker() == "Sigma" && cc && cc->has_rawCrop()) { // foveon images
                 int lm, tm, w, h;
                 cc->get_rawCrop(lm, tm, w, h);
                 left_margin = lm;
diff --git a/rtengine/rtthumbnail.cc b/rtengine/rtthumbnail.cc
index 798964275..2854df91a 100644
--- a/rtengine/rtthumbnail.cc
+++ b/rtengine/rtthumbnail.cc
@@ -544,20 +544,32 @@ Thumbnail* Thumbnail::loadFromRaw (const Glib::ustring& fname, RawMetaDataLocati
             int left_margin = ri->get_leftmargin();
             firstgreen += left_margin;
             int top_margin = ri->get_topmargin();
+            int wmax = tmpw;
+            int hmax = tmph;
             if(ri->get_maker() == "Sigma" && ri->DNGVERSION()) { // Hack to prevent sigma dng files from crashing
-                tmpw = (width - 2 - left_margin) / hskip;
-                tmph = (height - 2 - top_margin) / vskip;
+                wmax = (width - 2 - left_margin) / hskip;
+                hmax = (height - 2 - top_margin) / vskip;
             }
 
-            for (int row = 1 + top_margin, y = 0; row < iheight + top_margin  - 1 && y < tmph; row += vskip, y++) {
+            int y = 0;
+            for (int row = 1 + top_margin; row < iheight + top_margin  - 1 && y < hmax; row += vskip, y++) {
                 rofs = row * iwidth;
 
-                for (int col = firstgreen, x = 0; col < iwidth + left_margin - 1 && x < tmpw; col += hskip, x++) {
+                int x = 0;
+                for (int col = firstgreen; col < iwidth + left_margin - 1 && x < wmax; col += hskip, x++) {
                     int ofs = rofs + col;
                     tmpImg->r(y, x) = image[ofs][0];
                     tmpImg->g(y, x) = image[ofs][1];
                     tmpImg->b(y, x) = image[ofs][2];
                 }
+                for (; x < tmpw; ++x) {
+                    tmpImg->r(y, x) = tmpImg->g(y, x) = tmpImg->b(y, x) = 0;
+                }
+            }
+            for (; y < tmph; ++y) {
+                for (int x = 0; x < tmpw; ++x) {
+                    tmpImg->r(y, x) = tmpImg->g(y, x) = tmpImg->b(y, x) = 0;
+                }
             }
         }
     }