diff --git a/src/cmd/internal/gc/fmt.go b/src/cmd/internal/gc/fmt.go
index 1a991a0a65..4e3045a929 100644
--- a/src/cmd/internal/gc/fmt.go
+++ b/src/cmd/internal/gc/fmt.go
@@ -1127,7 +1127,7 @@ func exprfmt(n *Node, prec int) string {
 		// Special case: name used as local variable in export.
 	// _ becomes ~b%d internally; print as _ for export
 	case ONAME:
-		if fmtmode == FExp && n.Sym != nil && n.Sym.Name[0] == '~' && n.Sym.Name[1] == 'b' {
+		if (fmtmode == FExp || fmtmode == FErr) && n.Sym != nil && n.Sym.Name[0] == '~' && n.Sym.Name[1] == 'b' {
 			return "_"
 		}
 		if fmtmode == FExp && n.Sym != nil && !isblank(n) && n.Vargen > 0 {
diff --git a/test/fixedbugs/issue9521.go b/test/fixedbugs/issue9521.go
new file mode 100644
index 0000000000..51b5204e7a
--- /dev/null
+++ b/test/fixedbugs/issue9521.go
@@ -0,0 +1,16 @@
+// errorcheck
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that an incorrect use of the blank identifer is caught.
+// Does not compile.
+
+package main
+
+func f() (_, _ []int) { return }
+
+func main() {
+	_ = append(f()) // ERROR "cannot use _"
+}