diff --git a/src/nix/build.cc b/src/nix/build.cc
index d6a6a8071..07b6b2f02 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -3,7 +3,6 @@
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
-#include "primops/flake.hh"
 
 using namespace nix;
 
diff --git a/src/nix/command.hh b/src/nix/command.hh
index a52fbb9ba..6d43261ac 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -108,6 +108,8 @@ struct InstallablesCommand : virtual Args, SourceExprCommand
 
     void prepare() override;
 
+    virtual bool useDefaultInstallables() { return true; }
+
 private:
 
     std::vector<std::string> _installables;
@@ -119,14 +121,14 @@ struct InstallableCommand : virtual Args, SourceExprCommand
 
     InstallableCommand()
     {
-        expectArg("installable", &_installable);
+        expectArg("installable", &_installable, true);
     }
 
     void prepare() override;
 
 private:
 
-    std::string _installable;
+    std::string _installable{"."};
 };
 
 /* A command that operates on zero or more store paths. */
@@ -146,6 +148,8 @@ public:
     virtual void run(ref<Store> store, Paths storePaths) = 0;
 
     void run(ref<Store> store) override;
+
+    bool useDefaultInstallables() override { return !all; }
 };
 
 /* A command that operates on exactly one store path. */
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index 9d87c70c3..c3ca87aa7 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -334,6 +334,10 @@ PathSet toDerivations(ref<Store> store,
 
 void InstallablesCommand::prepare()
 {
+    if (_installables.empty() && !file && useDefaultInstallables())
+        // FIXME: commands like "nix install" should not have a
+        // default, probably.
+        _installables.push_back(".");
     installables = parseInstallables(getStore(), _installables);
 }