diff --git a/dlib/image_transforms/colormaps.h b/dlib/image_transforms/colormaps.h
index 40d9c335c..258aaf3cb 100644
--- a/dlib/image_transforms/colormaps.h
+++ b/dlib/image_transforms/colormaps.h
@@ -145,7 +145,10 @@ namespace dlib
     )
     {
         typedef op_heatmap<image_type> op;
-        return matrix_op<op>(op(img,max(mat(img)),min(mat(img))));
+        if (img.size() != 0)
+            return matrix_op<op>(op(img,max(mat(img)),min(mat(img))));
+        else
+            return matrix_op<op>(op(img,0,0));
     }
 
 // ----------------------------------------------------------------------------------------
@@ -252,7 +255,10 @@ namespace dlib
     )
     {
         typedef op_jet<image_type> op;
-        return matrix_op<op>(op(img,max(mat(img)),min(mat(img))));
+        if (img.size() != 0)
+            return matrix_op<op>(op(img,max(mat(img)),min(mat(img))));
+        else
+            return matrix_op<op>(op(img,0,0));
     }
 
 // ----------------------------------------------------------------------------------------