diff --git a/generic_rag/graphs/ret_gen.py b/generic_rag/graphs/ret_gen.py
index c6f58d3..b3c97f4 100644
--- a/generic_rag/graphs/ret_gen.py
+++ b/generic_rag/graphs/ret_gen.py
@@ -48,10 +48,11 @@ class RetGenLangGraph:
         """
         Method that retrieves the PDF sources used during the last invoke.
         """
-        if self.last_invoke is None:
-            return []
-
         pdf_sources = {}
+
+        if self.last_invoke is None:
+            return pdf_sources
+
         for context in self.last_invoke["context"]:
             try:
                 Path(context.metadata["source"]).suffix == ".pdf"
@@ -84,10 +85,11 @@ class RetGenLangGraph:
         """
         Method that retrieves the web sources used during the last invoke.
         """
-        if self.last_invoke is None:
-            return set()
-
         web_sources = set()
+
+        if self.last_invoke is None:
+            return web_sources
+
         for context in self.last_invoke["context"]:
             try:
                 context.metadata["filetype"] == "web"