diff --git a/swarm-builder/src/nix_build.rs b/swarm-builder/src/nix_build.rs
index a0649e3b..6a5721cb 100644
--- a/swarm-builder/src/nix_build.rs
+++ b/swarm-builder/src/nix_build.rs
@@ -237,14 +237,15 @@ impl Tail {
     }
 
     /// The kept bytes. When the stream was longer than `keep`, the cut line
-    /// at the front is dropped, so the result starts on a line boundary
-    /// (unless it holds no newline at all).
+    /// at the front is dropped, so the result starts on a line boundary,
+    /// unless that line is all the window holds.
     fn finish(mut self) -> Vec<u8> {
         if self.buf.len() > self.keep {
             self.trim();
         }
         if self.cut
             && let Some(nl) = self.buf.iter().position(|&b| b == b'\n')
+            && nl + 1 < self.buf.len()
         {
             self.buf.drain(..=nl);
         }
@@ -566,6 +567,10 @@ mod tests {
         assert_eq!(String::from_utf8(cut).expect("utf-8"), "cccc\ndddd\n");
         let one_long_line = read_tail(&b"x".repeat(50)[..], 12).await.expect("read");
         assert_eq!(one_long_line, b"x".repeat(12));
+        let long_last_line = read_tail(format!("{}\n", "x".repeat(50)).as_bytes(), 12)
+            .await
+            .expect("read");
+        assert_eq!(long_last_line, format!("{}\n", "x".repeat(11)).as_bytes());
     }
 
     #[test]