diff --git a/pkgs/development/beam-modules/build-mix.nix b/pkgs/development/beam-modules/build-mix.nix
index dd2d3f7dea97..243028f16365 100644
--- a/pkgs/development/beam-modules/build-mix.nix
+++ b/pkgs/development/beam-modules/build-mix.nix
@@ -124,7 +124,7 @@ let
             # phoenix applications need the source of phoenix and phoenix_html to
             # build javascript and css assets.
             mkdir -p $out/src
-            cp -r $src/* "$out/src"
+            cp -r "$src/." "$out/src"
 
             runHook postInstall
           '';