diff --git a/build.go b/build.go
index bef9f14..6f3398c 100644
--- a/build.go
+++ b/build.go
@@ -2,6 +2,7 @@ package nixbuild
 
 import (
 	"context"
+	"io"
 	"iter"
 	"strings"
 )
@@ -24,12 +25,10 @@ func Build(ctx context.Context, installables iter.Seq[string]) error {
 		cmd.Args = append(cmd.Args, FlagVerbose)
 	}
 
-	stdin, err := cmd.StdinPipe()
-	if err != nil {
-		return err
-	}
+	ir, iw := io.Pipe()
+	cmd.Stdin = ir
 
-	if err = cmd.Start(); err != nil {
+	if err := cmd.Start(); err != nil {
 		return err
 	}
 
@@ -38,12 +37,12 @@ func Build(ctx context.Context, installables iter.Seq[string]) error {
 			// this is just what nix requires now :c
 			drv += "^*"
 		}
-		if _, err = stdin.Write([]byte(drv + "\n")); err != nil {
+		if _, err := iw.Write([]byte(drv + "\n")); err != nil {
 			return err
 		}
 	}
 
-	if err = stdin.Close(); err != nil {
+	if err := iw.Close(); err != nil {
 		return err
 	}
 
diff --git a/instantiated.go b/instantiated.go
index 44e4696..6d3651f 100644
--- a/instantiated.go
+++ b/instantiated.go
@@ -8,10 +8,9 @@ import (
 	"iter"
 	"os/exec"
 	"path"
-	"runtime"
 	"slices"
 	"strings"
-	"syscall"
+	"sync"
 )
 
 const (
@@ -132,16 +131,18 @@ func (d *InstantiatedDecoder) Decode() ([]string, error) {
 type InstantiatedEvaluator struct {
 	// underlying nix program
 	cmd *exec.Cmd
-	// kills the nix command
-	cancel context.CancelFunc
 	// populated by Close
 	waitErr error
+	// synchronises access to waitErr
+	waitMu sync.RWMutex
 
 	*InstantiatedDecoder
 }
 
-// Err returns the first error encountered by the [InstantiatedEvaluator].
+// Err blocks until process exit and returns the first error encountered by the [InstantiatedEvaluator].
 func (e *InstantiatedEvaluator) Err() error {
+	e.waitMu.RLock()
+	defer e.waitMu.RUnlock()
 	return errors.Join(e.waitErr, e.InstantiatedDecoder.Err())
 }
 
@@ -157,41 +158,26 @@ func NewInstantiatedEvaluator(ctx context.Context, installable string) (*Instant
 			// increase verbosity so nix outputs 'instantiated' messages
 			FlagPrintBuildLogs, FlagDebug,
 		),
-		cancel: cancel,
 	}
 
 	e.cmd.Stdout = Stdout
 
 	// verbose output ends up on stderr in the current nix implementation
-	if stderr, err := e.cmd.StderrPipe(); err != nil {
-		cancel()
-		return nil, err
-	} else {
-		e.InstantiatedDecoder = NewInstantiatedDecoder(stderr)
-	}
+	er, ew := io.Pipe()
+	e.cmd.Stderr = ew
+	e.InstantiatedDecoder = NewInstantiatedDecoder(er)
 
 	if err := e.cmd.Start(); err != nil {
 		cancel()
 		// have finalizer take care of the pipe
 		return nil, err
 	}
+	e.waitMu.Lock()
+	go func() { e.waitErr = e.cmd.Wait(); cancel(); _ = ew.Close(); e.waitMu.Unlock() }()
 
-	runtime.SetFinalizer(e, (*InstantiatedEvaluator).Close)
 	return e, nil
 }
 
-// Close releases system resources held by [InstantiatedEvaluator].
-func (e *InstantiatedEvaluator) Close() error {
-	if e.cmd.ProcessState != nil {
-		return syscall.EBADE
-	}
-
-	e.cancel()
-	e.waitErr = e.cmd.Wait()
-	runtime.SetFinalizer(e, nil)
-	return e.Err()
-}
-
 // EvalInstantiated calls the underlying [InstantiatedDecoder.Decode] of a new [InstantiatedEvaluator].
 func EvalInstantiated(ctx context.Context, installable string) ([]string, error) {
 	evaluator, err := NewInstantiatedEvaluator(ctx, installable)
@@ -199,5 +185,5 @@ func EvalInstantiated(ctx context.Context, installable string) ([]string, error)
 		return nil, err
 	}
 	instantiated, _ := evaluator.Decode() // error joined by Close
-	return instantiated, evaluator.Close()
+	return instantiated, evaluator.Err()
 }