diff --git a/cli/error_plan9.go b/cli/error_plan9.go
new file mode 100644
index 00000000..78abf760
--- /dev/null
+++ b/cli/error_plan9.go
@@ -0,0 +1,8 @@
+package cli
+
+import "os"
+
+func isErrnoNotSupported(err error) bool {
+	// Sync on os.Stdin or os.Stderr returns "permission denied".
+	return os.IsPermission(err)
+}
diff --git a/cli/error_posix.go b/cli/error_posix.go
index d2d6b2bb..7ab44166 100644
--- a/cli/error_posix.go
+++ b/cli/error_posix.go
@@ -1,4 +1,4 @@
-//+build !windows
+//+build !windows,!plan9
 
 package cli