diff --git a/header.go b/header.go
index 72cee93..bc1a5e5 100644
--- a/header.go
+++ b/header.go
@@ -5,6 +5,7 @@ package rtcp
 
 import (
 	"encoding/binary"
+	"fmt"
 )
 
 // PacketType specifies the type of an RTCP packet.
@@ -59,7 +60,7 @@ func (p PacketType) String() string {
 	case TypeExtendedReport:
 		return "XR"
 	default:
-		return string(p)
+		return fmt.Sprintf("invalid value %d", uint8(p))
 	}
 }
 
diff --git a/header_test.go b/header_test.go
index 910c9d5..5fdcb50 100644
--- a/header_test.go
+++ b/header_test.go
@@ -105,3 +105,17 @@ func TestHeaderRoundTrip(t *testing.T) {
 		assert.Equalf(t, test.Header, decoded, "%q header round trip mismatch", test.Name)
 	}
 }
+
+func TestPacketTypeString(t *testing.T) {
+	for _, test := range []struct {
+		Type PacketType
+		Want string
+	}{
+		{TypeSenderReport, "SR"},
+		{TypeExtendedReport, "XR"},
+		{PacketType(65), "invalid value 65"},
+		{PacketType(0), "invalid value 0"},
+	} {
+		assert.Equal(t, test.Want, test.Type.String())
+	}
+}
diff --git a/source_description.go b/source_description.go
index 951bf8f..2f79b89 100644
--- a/source_description.go
+++ b/source_description.go
@@ -49,7 +49,7 @@ func (s SDESType) String() string {
 	case SDESPrivate:
 		return "PRIV"
 	default:
-		return string(s)
+		return fmt.Sprintf("invalid value %d", uint8(s))
 	}
 }
 
diff --git a/source_description_test.go b/source_description_test.go
index eccf75b..3221550 100644
--- a/source_description_test.go
+++ b/source_description_test.go
@@ -375,3 +375,17 @@ func TestSourceDescriptionRoundTripMaxLength(t *testing.T) {
 	assert.NoError(t, decoded.Unmarshal(data))
 	assert.Equal(t, desc, decoded)
 }
+
+func TestSDESTypeString(t *testing.T) {
+	for _, test := range []struct {
+		Type SDESType
+		Want string
+	}{
+		{SDESCNAME, "CNAME"},
+		{SDESPrivate, "PRIV"},
+		{SDESType(65), "invalid value 65"},
+		{SDESType(200), "invalid value 200"},
+	} {
+		assert.Equal(t, test.Want, test.Type.String())
+	}
+}