diff --git a/src/nix/bundle.cc b/src/nix/bundle.cc
index 9fc2656bb..445db5d5f 100644
--- a/src/nix/bundle.cc
+++ b/src/nix/bundle.cc
@@ -69,39 +69,24 @@ struct CmdBundle : InstallableCommand
     {
         auto evalState = getEvalState();
 
-        auto app = installable->toApp(*evalState).resolve(getEvalStore(), store);
-
         auto [progFlakeRef, progName] = parseFlakeRefWithFragment(installable->what(), absPath("."));
         const flake::LockFlags lockFlagsProg{ .writeLockFile = false };
         auto programInstallable = InstallableFlake(this,
             evalState, std::move(progFlakeRef),
-            Strings{progName == "" ? "defaultPackage" : progName},
-            Strings({"packages."+settings.thisSystem.get()+".","legacyPackages."+settings.thisSystem.get()+"."}), lockFlagsProg);
+            Strings{progName == "" ? "defaultApp" : progName},
+            Strings(this->getDefaultFlakeAttrPathPrefixes()),
+            lockFlagsProg);
         auto val = programInstallable.toValue(*evalState).first;
 
         auto [bundlerFlakeRef, bundlerName] = parseFlakeRefWithFragment(bundler, absPath("."));
         const flake::LockFlags lockFlags{ .writeLockFile = false };
         auto bundler = InstallableFlake(this,
             evalState, std::move(bundlerFlakeRef),
-            Strings{bundlerName == "" ? "defaultBundler" : bundlerName},
+            Strings{bundlerName == "" ? "defaultBundler." + settings.thisSystem.get() : settings.thisSystem.get() + "." + bundlerName},
             Strings({"bundlers."}), lockFlags);
 
-        auto attrs = evalState->buildBindings(2);
-
-        Value & prog = *evalState->allocAttr(*arg, evalState->symbols.create("program"));
-        evalState->mkAttrs(prog,val->attrs->size());
-        for (auto &j : *(val->attrs)){
-            prog.attrs->push_back(j);
-        }
-        prog.attrs->sort();
-
-        attrs.alloc("system").mkString(settings.thisSystem.get());
-
         auto vRes = evalState->allocValue();
-        evalState->callFunction(
-            *bundler.toValue(*evalState).first,
-            evalState->allocValue()->mkAttrs(attrs),
-            *vRes, noPos);
+        evalState->callFunction(*bundler.toValue(*evalState).first, *val, *vRes, noPos);
 
         if (!evalState->isDerivation(*vRes))
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
@@ -123,9 +108,12 @@ struct CmdBundle : InstallableCommand
 
         auto outPathS = store->printStorePath(outPath);
 
-        if (!outLink)
-            outLink = baseNameOf(app.program);
+        if (!outLink) {
+            auto &attr = vRes->attrs->need(evalState->sName);
+            outLink = evalState->forceStringNoCtx(*attr.value,*attr.pos);
+        }
 
+        // TODO: will crash if not a localFSStore?
         store.dynamic_pointer_cast<LocalFSStore>()->addPermRoot(outPath, absPath(*outLink));
     }
 };
diff --git a/src/nix/bundle.md b/src/nix/bundle.md
index 5e2298376..317b7000b 100644
--- a/src/nix/bundle.md
+++ b/src/nix/bundle.md
@@ -18,12 +18,20 @@ R""(
   nix (Nix) 2.4pre20201215_e3ddffb
   ```
 
+* Bundle a Hello using a specific bundler:
+
+  ```console
+  # nix bundle --bundler github:NixOS/bundlers#toDockerImage nixpkgs#hello
+  # docker load < hello-2.10.tar.gz
+  # docker run hello-2.10:latest hello
+  Hello, world!
+  ```
+
 # Description
 
-`nix bundle` packs the closure of the [Nix app](./nix3-run.md)
-*installable* into a single self-extracting executable. See the
-[`nix-bundle` homepage](https://github.com/matthewbauer/nix-bundle)
-for more details.
+`nix bundle`, by default, packs the closure of the [Nix app](./nix3-run.md)
+*installable* into a single self-extracting executable. See the [`nix-bundle`
+homepage](https://github.com/matthewbauer/nix-bundle) for more details.
 
 > **Note**
 >
@@ -31,6 +39,29 @@ for more details.
 
 # Bundler definitions
 
-TODO
+If no flake output attribute is given, `nix bundle` tries the following
+flake output attributes:
+
+* `defaultBundler.<system>`
+
+If an attribute *name* is given, `nix run` tries the following flake
+output attributes:
+
+* `bundler.<system>.<name>`
+
+# Bundlers
+
+An bundlers is specified by a flake output attribute named
+`bundlers.<system>.<name>` or `defaultBundler.<system>`. It looks like this:
+
+```nix
+bundlers.x86_64-linux.identity = drv: drv;
+
+bundlers.x86_64-linux.blender_2_79 = drv: self.packages.x86_64-linux.blender_2_79;
+
+defaultBundler.x86_64-linux = drv: drv;
+```
+
+A bundler must be a function that accepts a derivation and returns a derivation.
 
 )""
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 97f4d911c..19bf9ab51 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -475,10 +475,7 @@ struct CmdFlakeCheck : FlakeCommand
                 state->forceValue(v, pos);
                 if (!v.isLambda())
                     throw Error("bundler must be a function");
-                if (!v.lambda.fun->formals ||
-                    !v.lambda.fun->formals->argNames.count(state->symbols.create("program")) ||
-                    !v.lambda.fun->formals->argNames.count(state->symbols.create("system")))
-                    throw Error("bundler must take formal arguments 'program' and 'system'");
+                // TODO: check types of inputs/outputs?
             } catch (Error & e) {
                 e.addTrace(pos, hintfmt("while checking the template '%s'", attrPath));
                 reportError(e);
@@ -609,14 +606,27 @@ struct CmdFlakeCheck : FlakeCommand
                                     *attr.value, *attr.pos);
                         }
 
-                        else if (name == "defaultBundler")
-                            checkBundler(name, vOutput, pos);
+                        else if (name == "defaultBundler") {
+                            state->forceAttrs(vOutput, pos);
+                            for (auto & attr : *vOutput.attrs) {
+                                checkSystemName(attr.name, *attr.pos);
+                                checkBundler(
+                                    fmt("%s.%s", name, attr.name),
+                                    *attr.value, *attr.pos);
+                            }
+                        }
 
                         else if (name == "bundlers") {
                             state->forceAttrs(vOutput, pos);
-                            for (auto & attr : *vOutput.attrs)
-                                checkBundler(fmt("%s.%s", name, attr.name),
-                                    *attr.value, *attr.pos);
+                            for (auto & attr : *vOutput.attrs) {
+                                checkSystemName(attr.name, *attr.pos);
+                                state->forceAttrs(*attr.value, *attr.pos);
+                                for (auto & attr2 : *attr.value->attrs) {
+                                    checkBundler(
+                                        fmt("%s.%s.%s", name, attr.name, attr2.name),
+                                        *attr2.value, *attr2.pos);
+                                }
+                            }
                         }
 
                         else