diff --git a/test/fixedbugs/issue12621.go b/test/fixedbugs/issue12621.go
new file mode 100644
index 0000000000..379a362d73
--- /dev/null
+++ b/test/fixedbugs/issue12621.go
@@ -0,0 +1,20 @@
+// run
+
+// Copyright 2018 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.
+
+// Issues 12576 and 12621: Negative untyped floating point constants
+// with small magnitude round to 0, not negative zero.
+
+package main
+
+import "math"
+
+var m = -1e-10000
+
+func main() {
+	if math.Signbit(m) {
+		panic(m)
+	}
+}