diff --git a/rflx/specification/parser.py b/rflx/specification/parser.py
index 7e00510d04..8f68cf122c 100644
--- a/rflx/specification/parser.py
+++ b/rflx/specification/parser.py
@@ -1190,7 +1190,7 @@ def __init__(self, skip_verification: bool = False, cached: bool = False) -> Non
         self.__cache = Cache(cached)
 
     def __convert_unit(
-        self, spec: Specification, filename: Path = STDIN, transitions: List[ID] = None
+        self, spec: Specification, filename: Path, transitions: List[ID] = None
     ) -> RecordFluxError:
         transitions = transitions or []
         error = RecordFluxError()
@@ -1254,7 +1254,7 @@ def parse_string(
         error = RecordFluxError()
         unit = AnalysisContext().get_from_buffer("<stdin>", string, rule=rule)
         if not diagnostics_to_error(unit.diagnostics, error):
-            error = self.__convert_unit(unit.root)
+            error = self.__convert_unit(unit.root, STDIN)
         error.propagate()
 
     def create_model(self) -> model.Model: