diff --git a/src/queue.rs b/src/queue.rs
index 8f9d63d..4db1654 100644
--- a/src/queue.rs
+++ b/src/queue.rs
@@ -132,10 +132,11 @@ where
         }
 
         // Try the size hint.
-        if let Some(val) = self.current.size_hint().1 {
-            if val < THRESHOLD && val != 0 {
-                return Some(val);
-            }
+        let (lower_bound, _) = self.current.size_hint();
+        // The iterator default implementation just returns 0.
+        // That's a problematic value, so skip it.
+        if lower_bound > 0 {
+            return Some(lower_bound);
         }
 
         // Otherwise we use the constant value.
diff --git a/src/source/done.rs b/src/source/done.rs
index fb26713..d908d96 100644
--- a/src/source/done.rs
+++ b/src/source/done.rs
@@ -57,6 +57,10 @@ where
         }
         next
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.input.size_hint()
+    }
 }
 
 impl<I> Source for Done<I>