diff --git a/src/os/exec_unix.go b/src/os/exec_unix.go
index 8d99b553422..ba6146ada11 100644
--- a/src/os/exec_unix.go
+++ b/src/os/exec_unix.go
@@ -103,6 +103,9 @@ func (p *Process) signal(sig Signal) error {
 }
 
 func (p *Process) pidSignal(s syscall.Signal) error {
+	if p.Pid == pidReleased {
+		return errors.New("os: process already released")
+	}
 	if p.Pid == pidUnset {
 		return errors.New("os: process not initialized")
 	}
diff --git a/src/os/exec_unix_test.go b/src/os/exec_unix_test.go
index 81d8e1cfee6..960f5d82187 100644
--- a/src/os/exec_unix_test.go
+++ b/src/os/exec_unix_test.go
@@ -40,7 +40,7 @@ func TestProcessAlreadyDone(t *testing.T) {
 		// EINVAL (see waitid in usr/src/uts/common/os/exit.c in
 		// illumos). This is configurable via sysconf(_SC_MAXPID), but
 		// we'll just take the default.
-		pid = 30000-1
+		pid = 30000 - 1
 	}
 
 	p, err := FindProcess(pid)
@@ -76,3 +76,14 @@ func TestUNIXProcessAlive(t *testing.T) {
 		t.Errorf("OS reported error for running process: %v", err)
 	}
 }
+
+func TestProcessBadPID(t *testing.T) {
+	p, err := FindProcess(-1)
+	if err != nil {
+		t.Fatalf("unexpected FindProcess error: %v", err)
+	}
+	err = p.Signal(syscall.Signal(0))
+	if err == nil {
+		t.Error("p.Signal succeeded unexpectedly")
+	}
+}