diff --git a/sphinx/builders/gettext.py b/sphinx/builders/gettext.py
index 0d7d0ac11d8..92edcc9f2dd 100644
--- a/sphinx/builders/gettext.py
+++ b/sphinx/builders/gettext.py
@@ -53,7 +53,10 @@ def add(self, msg: str, origin: Union[Element, "MsgOrigin"]) -> None:
         if msg not in self.metadata:  # faster lookup in hash
             self.messages.append(msg)
             self.metadata[msg] = []
-        self.metadata[msg].append((origin.source, origin.line, origin.uid))  # type: ignore
+        line = origin.line
+        if line is None:
+            line = -1
+        self.metadata[msg].append((origin.source, line, origin.uid))  # type: ignore
 
     def __iter__(self) -> Generator[Message, None, None]:
         for message in self.messages: