diff --git a/lib/trivial.nix b/lib/trivial.nix
index f85c74ab88e..21642ca0bdc 100644
--- a/lib/trivial.nix
+++ b/lib/trivial.nix
@@ -71,7 +71,7 @@ rec {
     + (if pathExists suffixFile then fileContents suffixFile else "pre-git");
 
   # Whether we're being called by nix-shell.
-  inNixShell = builtins.getEnv "IN_NIX_SHELL" == "1";
+  inNixShell = builtins.getEnv "IN_NIX_SHELL" != "";
 
   # Return minimum/maximum of two numbers.
   min = x: y: if x < y then x else y;
diff --git a/pkgs/development/tools/pypi2nix/default.nix b/pkgs/development/tools/pypi2nix/default.nix
index 02ab4c06a50..2884b23049a 100644
--- a/pkgs/development/tools/pypi2nix/default.nix
+++ b/pkgs/development/tools/pypi2nix/default.nix
@@ -36,7 +36,7 @@ in stdenv.mkDerivation rec {
     # mv effect-*/effect                 $out/pkgs/effect
     mv requests-*/requests              $out/pkgs/
 
-    if [ "$IN_NIX_SHELL" != "1" ]; then
+    if [ -z "$IN_NIX_SHELL" ]; then
       if [ -e git-export ]; then
         mv git-export/src/pypi2nix      $out/pkgs/pypi2nix
       else