diff --git a/src/pkg/os/exec.go b/src/pkg/os/exec.go
index e2234f14a06..40e6c1774ee 100644
--- a/src/pkg/os/exec.go
+++ b/src/pkg/os/exec.go
@@ -13,10 +13,11 @@ import (
 type Process struct {
 	Pid    int
 	handle int
+	done   bool // process has been successfuly waited on
 }
 
 func newProcess(pid, handle int) *Process {
-	p := &Process{pid, handle}
+	p := &Process{Pid: pid, handle: handle}
 	runtime.SetFinalizer(p, (*Process).Release)
 	return p
 }
diff --git a/src/pkg/os/exec_unix.go b/src/pkg/os/exec_unix.go
index cf5ea9b617e..8a4b2e1b806 100644
--- a/src/pkg/os/exec_unix.go
+++ b/src/pkg/os/exec_unix.go
@@ -38,6 +38,9 @@ func (p *Process) Wait(options int) (w *Waitmsg, err Error) {
 	if e != 0 {
 		return nil, NewSyscallError("wait", e)
 	}
+	if options&WSTOPPED == 0 {
+		p.done = true
+	}
 	w = new(Waitmsg)
 	w.Pid = pid1
 	w.WaitStatus = status
@@ -47,6 +50,9 @@ func (p *Process) Wait(options int) (w *Waitmsg, err Error) {
 
 // Signal sends a signal to the Process.
 func (p *Process) Signal(sig Signal) Error {
+	if p.done {
+		return NewError("os: process already finished")
+	}
 	if e := syscall.Kill(p.Pid, int(sig.(UnixSignal))); e != 0 {
 		return Errno(e)
 	}
diff --git a/src/pkg/os/exec_windows.go b/src/pkg/os/exec_windows.go
index 5b432d39884..65e94ac4acc 100644
--- a/src/pkg/os/exec_windows.go
+++ b/src/pkg/os/exec_windows.go
@@ -24,11 +24,15 @@ func (p *Process) Wait(options int) (w *Waitmsg, err Error) {
 	if e != 0 {
 		return nil, NewSyscallError("GetExitCodeProcess", e)
 	}
+	p.done = true
 	return &Waitmsg{p.Pid, syscall.WaitStatus{s, ec}, new(syscall.Rusage)}, nil
 }
 
 // Signal sends a signal to the Process.
 func (p *Process) Signal(sig Signal) Error {
+	if p.done {
+		return NewError("os: process already finished")
+	}
 	switch sig.(UnixSignal) {
 	case SIGKILL:
 		e := syscall.TerminateProcess(syscall.Handle(p.handle), 1)
diff --git a/src/pkg/os/os_test.go b/src/pkg/os/os_test.go
index c22b536d557..dadae03e8cf 100644
--- a/src/pkg/os/os_test.go
+++ b/src/pkg/os/os_test.go
@@ -895,7 +895,14 @@ func run(t *testing.T, cmd []string) string {
 
 	var b bytes.Buffer
 	io.Copy(&b, r)
-	p.Wait(0)
+	_, err = p.Wait(0)
+	if err != nil {
+		t.Fatalf("run hostname Wait: %v", err)
+	}
+	err = p.Kill()
+	if err == nil {
+		t.Errorf("expected an error from Kill running 'hostname'")
+	}
 	output := b.String()
 	if n := len(output); n > 0 && output[n-1] == '\n' {
 		output = output[0 : n-1]