diff --git a/servant/src/Servant/Types/SourceT.hs b/servant/src/Servant/Types/SourceT.hs
index 84cb4b6a..d66405c5 100644
--- a/servant/src/Servant/Types/SourceT.hs
+++ b/servant/src/Servant/Types/SourceT.hs
@@ -212,7 +212,7 @@ instance (QC.Arbitrary a, Monad m) => QC.Arbitrary (StepT m a) where
 -- >>> source "foo" :: SourceT Identity Char
 -- fromStepT (Effect (Identity (Yield 'f' (Yield 'o' (Yield 'o' Stop)))))
 --
-source :: [a] -> SourceT m a
+source :: Foldable f => f a -> SourceT m a
 source = fromStepT . foldr Yield Stop
 
 -- | Get the answers.