diff --git a/common/flatpak-image-source-private.h b/common/flatpak-image-source-private.h
index beca07b0..42f1a5bb 100644
--- a/common/flatpak-image-source-private.h
+++ b/common/flatpak-image-source-private.h
@@ -32,6 +32,12 @@ G_DECLARE_FINAL_TYPE (FlatpakImageSource,
                       FLATPAK, IMAGE_SOURCE,
                       GObject)
 
+FlatpakImageSource *flatpak_image_source_new (FlatpakOciRegistry *registry,
+                                              const char         *repository,
+                                              const char         *digest,
+                                              GCancellable       *cancellable,
+                                              GError            **error);
+
 FlatpakImageSource *flatpak_image_source_new_local (GFile        *file,
                                                     const char   *reference,
                                                     GCancellable *cancellable,
diff --git a/common/flatpak-image-source.c b/common/flatpak-image-source.c
index 3844d37e..ef29a976 100644
--- a/common/flatpak-image-source.c
+++ b/common/flatpak-image-source.c
@@ -68,7 +68,7 @@ flatpak_image_source_init (FlatpakImageSource *self)
 {
 }
 
-static FlatpakImageSource *
+FlatpakImageSource *
 flatpak_image_source_new (FlatpakOciRegistry *registry,
                           const char         *repository,
                           const char         *digest,
@@ -78,6 +78,12 @@ flatpak_image_source_new (FlatpakOciRegistry *registry,
   g_autoptr(FlatpakImageSource) self = NULL;
   g_autoptr(FlatpakOciVersioned) versioned = NULL;
 
+  if (!g_str_has_prefix (digest, "sha256:"))
+    {
+      flatpak_fail_error (error, FLATPAK_ERROR_INVALID_DATA, _("Only sha256 image checksums are supported"));
+      return NULL;
+    }
+
   self = g_object_new (FLATPAK_TYPE_IMAGE_SOURCE, NULL);
   self->registry = g_object_ref (registry);
   self->repository = g_strdup (repository);