2 files changed, 26 insertions, 6 deletions
diff --git a/src/runtime/string.go b/src/runtime/string.go
index 112ce5d588..ef28ba9828 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -139,7 +139,8 @@ func slicebytetostringtmp(b []byte) string {
 func stringtoslicebyte(buf *tmpBuf, s string) []byte {
 	var b []byte
 	if buf != nil && len(s) <= len(buf) {
-		b = buf[:len(s):len(s)]
+		*buf = tmpBuf{}
+		b = buf[:len(s)]
 	} else {
 		b = rawbyteslice(len(s))
 	}
@@ -171,7 +172,8 @@ func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
 	}
 	var a []rune
 	if buf != nil && n <= len(buf) {
-		a = buf[:n:n]
+		*buf = [tmpStringBufSize]rune{}
+		a = buf[:n]
 	} else {
 		a = rawruneslice(n)
 	}
diff --git a/src/runtime/string_test.go b/src/runtime/string_test.go
index ee9709e87d..0f1d82a481 100644
--- a/src/runtime/string_test.go
+++ b/src/runtime/string_test.go
@@ -238,17 +238,35 @@ func TestRangeStringCast(t *testing.T) {
 	}
 }
 
+func isZeroed(b []byte) bool {
+	for _, x := range b {
+		if x != 0 {
+			return false
+		}
+	}
+	return true
+}
+
+func isZeroedR(r []rune) bool {
+	for _, x := range r {
+		if x != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 func TestString2Slice(t *testing.T) {
 	// Make sure we don't return slices that expose
 	// an unzeroed section of stack-allocated temp buf
 	// between len and cap. See issue 14232.
 	s := "foož"
 	b := ([]byte)(s)
-	if cap(b) != 5 {
-		t.Errorf("want cap of 5, got %d", cap(b))
+	if !isZeroed(b[len(b):cap(b)]) {
+		t.Errorf("extra bytes not zeroed")
 	}
 	r := ([]rune)(s)
-	if cap(r) != 4 {
-		t.Errorf("want cap of 4, got %d", cap(r))
+	if !isZeroedR(r[len(r):cap(r)]) {
+		t.Errorf("extra runes not zeroed")
 	}
 }