diff --git a/mutalyzer/Retriever.py b/mutalyzer/Retriever.py
index 99aedd9722f600ab5ca6d8943d3161feb248a2bd..613bd789f589639ee8fe05269d55a6d01385b96a 100644
--- a/mutalyzer/Retriever.py
+++ b/mutalyzer/Retriever.py
@@ -720,6 +720,10 @@ class GenBankRetriever(Retriever):
         """
         if (identifier[0].isdigit()) : # This is a GI identifier.
             name = self._database.getGBFromGI(identifier)
+            if name is None:
+                self._output.addMessage(__file__, 4, "ERETR",
+                                        "Unknown reference: %s" % identifier)
+                return
         else :
             name = identifier