diff --git a/libheif/image-items/image_item.cc b/libheif/image-items/image_item.cc
index e5c655ee89..baf71af659 100644
--- a/libheif/image-items/image_item.cc
+++ b/libheif/image-items/image_item.cc
@@ -69,11 +69,19 @@ std::shared_ptr<HeifFile> ImageItem::get_file() const
 
 heif_property_id ImageItem::add_property(std::shared_ptr<Box> property, bool essential)
 {
+  // TODO: is this correct? What happens when add_property does deduplicate the property?
   m_properties.push_back(property);
   return get_file()->add_property(get_id(), property, essential);
 }
 
 
+heif_property_id ImageItem::add_property_without_deduplication(std::shared_ptr<Box> property, bool essential)
+{
+  m_properties.push_back(property);
+  return get_file()->add_property_without_deduplication(get_id(), property, essential);
+}
+
+
 Error ImageItem::init_decoder_from_item(heif_item_id id)
 {
   m_id = id;
diff --git a/libheif/image-items/image_item.h b/libheif/image-items/image_item.h
index d13d565bdc..2d222a4fdb 100644
--- a/libheif/image-items/image_item.h
+++ b/libheif/image-items/image_item.h
@@ -114,6 +114,8 @@ class ImageItem : public ErrorBuffer
 
   heif_property_id add_property(std::shared_ptr<Box> property, bool essential);
 
+  heif_property_id add_property_without_deduplication(std::shared_ptr<Box> property, bool essential);
+
   void set_resolution(uint32_t w, uint32_t h)
   {
     m_width = w;
diff --git a/libheif/image-items/unc_image.cc b/libheif/image-items/unc_image.cc
index 10593f98a2..98adbd5a11 100644
--- a/libheif/image-items/unc_image.cc
+++ b/libheif/image-items/unc_image.cc
@@ -336,19 +336,19 @@ Result<std::shared_ptr<ImageItem_uncompressed>> ImageItem_uncompressed::add_unci
   assert(headers.uncC);
 
   if (headers.uncC) {
-    file->add_property(unci_id, headers.uncC, true);
+    unci_image->add_property(headers.uncC, true);
   }
 
   if (headers.cmpd) {
-    file->add_property(unci_id, headers.cmpd, true);
+    unci_image->add_property(headers.cmpd, true);
   }
 
   // Add `ispe` property
 
-  file->add_ispe_property(unci_id,
-                          static_cast<uint32_t>(parameters->image_width),
-                          static_cast<uint32_t>(parameters->image_height),
-                          true);
+  auto ispe = std::make_shared<Box_ispe>();
+  ispe->set_size(static_cast<uint32_t>(parameters->image_width),
+                 static_cast<uint32_t>(parameters->image_height));
+  unci_image->add_property(ispe, true);
 
   if (parameters->compression != heif_unci_compression_off) {
     auto icef = std::make_shared<Box_icef>();
@@ -374,8 +374,8 @@ Result<std::shared_ptr<ImageItem_uncompressed>> ImageItem_uncompressed::add_unci
       assert(false);
     }
 
-    file->add_property(unci_id, cmpC, true);
-    file->add_property_without_deduplication(unci_id, icef, true); // icef is empty. A normal add_property() would lead to a wrong deduplication.
+    unci_image->add_property(cmpC, true);
+    unci_image->add_property_without_deduplication(icef, true); // icef is empty. A normal add_property() would lead to a wrong deduplication.
   }
 
   // Create empty image. If we use compression, we append the data piece by piece.