diff --git a/repo2docker/buildpacks/nix/nix-shell-wrapper b/repo2docker/buildpacks/nix/nix-shell-wrapper
index 7490d78e..e9c8d71a 100644
--- a/repo2docker/buildpacks/nix/nix-shell-wrapper
+++ b/repo2docker/buildpacks/nix/nix-shell-wrapper
@@ -8,38 +8,38 @@ _term() {
 
 trap _term SIGTERM
 
+# if there is a binder/ sub-directory it takes precedence
+# files outside it are ignored
 # find binder sub-directory (if present)
-for dir in "./.binder" "./binder"; do
+binder_dir="./"
+for dir in "./binder" "./.binder" ; do
   if [ -e $dir ]; then
     binder_dir=$dir
+    break
   fi
 done
 
-# if there is a binder/ sub-directory it takes precedence
-# files outside it are ignored
-if [ -z $binder_dir ]; then
-  nixpath="$binder_dir/default.nix";
-  if [ -f $binder_dir/start ]; then
-    chmod u+x $binder_dir/start
-    # Using `$@`` here which is what the internet recommends leads to
-    # errors when the command is something like `jupyter --ip=0.0.0.0 ...`
-    # as nix-shell picks that up as an argument to it instead of the command.
-    # There are several issues on the nix repos discussing this and adding support
-    # for -- to indicate "all arguments after this are for the command, not nix-shell"
-    # but it seems they have stalled/not yet produced an implementation.
-    # So let's use `$*` for now.
-    nix-shell $nixpath --command "$binder_dir/start $*" &
-  else
-    nix-shell $nixpath --command "$*" &
-  fi
+# raise error if both binder and .binder are found
+if [[ -d "./binder" && -d "./.binder" ]]; then
+  echo "Error: Found both binder and .binder directories."
+  exit 1
+fi
+
+echo "binder_dir is: $binder_dir"
+
+nixpath="$binder_dir/default.nix";
+if [ -f $binder_dir/start ]; then
+  chmod u+x $binder_dir/start
+  # Using `$@`` here which is what the internet recommends leads to
+  # errors when the command is something like `jupyter --ip=0.0.0.0 ...`
+  # as nix-shell picks that up as an argument to it instead of the command.
+  # There are several issues on the nix repos discussing this and adding support
+  # for -- to indicate "all arguments after this are for the command, not nix-shell"
+  # but it seems they have stalled/not yet produced an implementation.
+  # So let's use `$*` for now.
+  nix-shell $nixpath --command "$binder_dir/start $*" &
 else
-  nixpath="./default.nix";
-  if [ -f ./start ]; then
-    chmod u+x ./start
-    nix-shell $nixpath --command "./start $*" &
-  else
-    nix-shell $nixpath --command "$*" &
-  fi
+  nix-shell $nixpath --command "$*" &
 fi
 
 PID=$!