diff --git a/pyteomics/proforma.py b/pyteomics/proforma.py
index 8049724..3a17f58 100644
--- a/pyteomics/proforma.py
+++ b/pyteomics/proforma.py
@@ -658,6 +658,8 @@ def __reduce__(self):
         return self.__class__, (self.value, self.extra, self.group_id, self.style), self.__getstate__()
 
     def __getstate__(self):
+        if self._definition is None:
+            return None
         state = self._definition.copy()
         state['source'] = None
         return state