diff --git a/WORKSPACE b/WORKSPACE
index 61b441cc1..ae756ff13 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -116,6 +116,20 @@ nixpkgs_package(
     repository = "@nixpkgs",
 )
 
+# This bazel build @output-derivation-is-a-file//... must fail
+# See https://github.com/tweag/rules_nixpkgs/issues/99 for details
+nixpkgs_package(
+    name = "output-derivation-is-a-file",
+    attribute_path = "",
+    nix_file_content = """let pkgs = import <nixpkgs> { config = {}; overlays = []; }; in pkgs.writeText "foo" "bar" """,
+    nix_file_deps = [
+        "//:nixpkgs.json",
+        "//:nixpkgs.nix",
+        "//tests:relative_imports/nixpkgs.nix",
+    ],
+    repository = "@nixpkgs",
+)
+
 nixpkgs_cc_configure(repository = "@remote_nixpkgs")
 
 nixpkgs_python_configure(repository = "@remote_nixpkgs")
diff --git a/nixpkgs/nixpkgs.bzl b/nixpkgs/nixpkgs.bzl
index 92d9fe7c9..0e9f4abd0 100644
--- a/nixpkgs/nixpkgs.bzl
+++ b/nixpkgs/nixpkgs.bzl
@@ -160,6 +160,14 @@ def _nixpkgs_package_impl(repository_ctx):
         )
         output_path = exec_result.stdout.splitlines()[-1]
 
+        # ensure that the output is a directory
+        test_path = repository_ctx.which("test")
+        _execute_or_fail(repository_ctx, [test_path, "-d", output_path],
+                         failure_message = "nixpkgs_package '@{}' outputs a single file which is not supported by rules_nixpkgs. Please only use directories.".format(
+                             repository_ctx.name
+                         ),
+        )
+
         # Build a forest of symlinks (like new_local_package() does) to the
         # Nix store.
         for target in _find_children(repository_ctx, output_path):