diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index 82a45c78a9e05..c0647d95a06cb 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
@@ -146,7 +146,7 @@ func growslice(et *_type, old slice, cap int) slice {
 	if cap > doublecap {
 		newcap = cap
 	} else {
-		if old.len < 1024 {
+		if old.cap < 1024 {
 			newcap = doublecap
 		} else {
 			// Check 0 < newcap to detect overflow
diff --git a/test/fixedbugs/issue41239.go b/test/fixedbugs/issue41239.go
new file mode 100644
index 0000000000000..3e9ef5eb66f4f
--- /dev/null
+++ b/test/fixedbugs/issue41239.go
@@ -0,0 +1,19 @@
+// run
+
+// Copyright 2020 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.
+
+package main
+
+import "fmt"
+
+func main() {
+	const N = 1024
+	var a [N]int
+	x := cap(append(a[:N-1:N], 9, 9))
+	y := cap(append(a[:N:N], 9))
+	if x != y {
+		panic(fmt.Sprintf("different capacity on append: %d vs %d", x, y))
+	}
+}