diff --git a/src/math/big/ratconv.go b/src/math/big/ratconv.go
index ac3c8bd..90053a9 100644
--- a/src/math/big/ratconv.go
+++ b/src/math/big/ratconv.go
@@ -169,6 +169,11 @@
 		n := exp5
 		if n < 0 {
 			n = -n
+			if n < 0 {
+				// This can occur if -n overflows. -(-1 << 63) would become
+				// -1 << 63, which is still negative.
+				return nil, false
+			}
 		}
 		if n > 1e6 {
 			return nil, false // avoid excessively large exponents
diff --git a/src/math/big/ratconv_test.go b/src/math/big/ratconv_test.go
index 15d206c..e55e655 100644
--- a/src/math/big/ratconv_test.go
+++ b/src/math/big/ratconv_test.go
@@ -104,6 +104,7 @@
 	{in: "4/3/"},
 	{in: "4/3."},
 	{in: "4/"},
+	{in: "13e-9223372036854775808"}, // CVE-2022-23772
 
 	// valid
 	{"0", "0", true},