diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index e59d7fe67..9a3b81bc9 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -559,11 +559,11 @@ static RegisterPrimOp primop_fetchTarball({
     .doc = R"(
       Download the specified URL, unpack it and return the path of the
       unpacked tree. The file must be a tape archive (`.tar`) compressed
-      with `gzip`, `bzip2` or `xz`. The top-level path component of the
-      files in the tarball is removed, so it is best if the tarball
-      contains a single directory at top level. The typical use of the
-      function is to obtain external Nix expression dependencies, such as
-      a particular version of Nixpkgs, e.g.
+      with `gzip`, `bzip2` or `xz`. If the tarball consists of a
+      single directory or non-executable file, then the top-level path
+      component of the files in the tarball is removed. The typical
+      use of the function is to obtain external Nix expression
+      dependencies, such as a particular version of Nixpkgs, e.g.
 
       ```nix
       with import (fetchTarball https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz) {};