diff --git a/sentry_sdk/tracing.py b/sentry_sdk/tracing.py
index 13fd04cf8e..cf708b839e 100644
--- a/sentry_sdk/tracing.py
+++ b/sentry_sdk/tracing.py
@@ -140,6 +140,7 @@ class TransactionSource(str, Enum):
     VIEW = "view"
 
     def __str__(self):
+        # type: () -> str
         return self.value