diff --git a/plugins/MergerSite/MergerSitePlugin.py b/plugins/MergerSite/MergerSitePlugin.py
index e7bdfa3d..af1a8ff5 100644
--- a/plugins/MergerSite/MergerSitePlugin.py
+++ b/plugins/MergerSite/MergerSitePlugin.py
@@ -235,7 +235,7 @@ class SiteStoragePlugin(object):
                     merged_site.log.error("[MISSING] %s" % content_inner_path)
                 # Data files in content.json
                 content_inner_path_dir = helper.getDirname(content_inner_path)  # Content.json dir relative to site
-                for file_relative_path in content["files"].keys():
+                for file_relative_path in content.get("files", {}).keys() + content.get("files_optional", {}).keys():
                     if not file_relative_path.endswith(".json"):
                         continue  # We only interesed in json files
                     file_inner_path = content_inner_path_dir + file_relative_path  # File Relative to site dir
diff --git a/src/Site/SiteStorage.py b/src/Site/SiteStorage.py
index 41744d7f..fd9d27e5 100644
--- a/src/Site/SiteStorage.py
+++ b/src/Site/SiteStorage.py
@@ -83,7 +83,7 @@ class SiteStorage(object):
                 self.log.error("[MISSING] %s" % content_inner_path)
             # Data files in content.json
             content_inner_path_dir = helper.getDirname(content_inner_path)  # Content.json dir relative to site
-            for file_relative_path in content["files"].keys():
+            for file_relative_path in content.get("files", {}).keys() + content.get("files_optional", {}).keys():
                 if not file_relative_path.endswith(".json"):
                     continue  # We only interesed in json files
                 file_inner_path = content_inner_path_dir + file_relative_path  # File Relative to site dir