diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index a12712d2540..b01158635f1 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -4007,9 +4007,12 @@ var convertTests = []struct {
 	{V(int16(-3)), V(string("\uFFFD"))},
 	{V(int32(-4)), V(string("\uFFFD"))},
 	{V(int64(-5)), V(string("\uFFFD"))},
+	{V(int64(-1 << 32)), V(string("\uFFFD"))},
+	{V(int64(1 << 32)), V(string("\uFFFD"))},
 	{V(uint(0x110001)), V(string("\uFFFD"))},
 	{V(uint32(0x110002)), V(string("\uFFFD"))},
 	{V(uint64(0x110003)), V(string("\uFFFD"))},
+	{V(uint64(1 << 32)), V(string("\uFFFD"))},
 	{V(uintptr(0x110004)), V(string("\uFFFD"))},
 
 	// named string
diff --git a/src/reflect/value.go b/src/reflect/value.go
index bf926a7453b..1f185b52e49 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -2681,12 +2681,20 @@ func cvtComplex(v Value, t Type) Value {
 
 // convertOp: intXX -> string
 func cvtIntString(v Value, t Type) Value {
-	return makeString(v.flag.ro(), string(rune(v.Int())), t)
+	s := "\uFFFD"
+	if x := v.Int(); int64(rune(x)) == x {
+		s = string(rune(x))
+	}
+	return makeString(v.flag.ro(), s, t)
 }
 
 // convertOp: uintXX -> string
 func cvtUintString(v Value, t Type) Value {
-	return makeString(v.flag.ro(), string(rune(v.Uint())), t)
+	s := "\uFFFD"
+	if x := v.Uint(); uint64(rune(x)) == x {
+		s = string(rune(x))
+	}
+	return makeString(v.flag.ro(), s, t)
 }
 
 // convertOp: []byte -> string