diff --git a/packet.go b/packet.go
index 6d2c7f3..a4c10f8 100644
--- a/packet.go
+++ b/packet.go
@@ -423,16 +423,20 @@ func (h *Header) SetExtension(id uint8, payload []byte) error { //nolint:gocogni
 		return nil
 	}
 
-	// No existing header extensions
-	h.Extension = true
+	// No existing header extensions. The one byte profile can only carry
+	// ids 1-14 and payloads of 1-16 bytes, everything else needs two bytes.
+	var profile uint16 = ExtensionProfileOneByte
+	if id > 14 || len(payload) == 0 || len(payload) > 16 {
+		profile = ExtensionProfileTwoByte
+	}
 
-	switch payloadLen := len(payload); {
-	case payloadLen <= 16:
-		h.ExtensionProfile = ExtensionProfileOneByte
-	case payloadLen > 16 && payloadLen < 256:
-		h.ExtensionProfile = ExtensionProfileTwoByte
+	// Don't mutate the header if Set is going to fail anyway
+	if err := headerExtensionCheck(profile, id, payload); err != nil {
+		return err
 	}
 
+	h.Extension = true
+	h.ExtensionProfile = profile
 	h.Extensions = append(h.Extensions, Extension{id: id, payload: payload})
 
 	return nil
diff --git a/packet_test.go b/packet_test.go
index a607449..11586c9 100644
--- a/packet_test.go
+++ b/packet_test.go
@@ -1708,3 +1708,48 @@ func BenchmarkUnmarshalHeader(b *testing.B) {
 		}
 	})
 }
+
+func TestSetExtensionFirstExtension(t *testing.T) {
+	t.Run("selects profile that can carry the extension", func(t *testing.T) {
+		cases := map[string]struct {
+			id      uint8
+			payload []byte
+			profile uint16
+		}{
+			"short payload": {1, []byte{0xAA}, ExtensionProfileOneByte},
+			"16 byte":       {14, make([]byte, 16), ExtensionProfileOneByte},
+			"17 byte":       {1, make([]byte, 17), ExtensionProfileTwoByte},
+			"255 byte":      {1, make([]byte, 255), ExtensionProfileTwoByte},
+			"id above 14":   {15, []byte{0xAA}, ExtensionProfileTwoByte},
+			"empty payload": {1, []byte{}, ExtensionProfileTwoByte},
+			"max id":        {255, []byte{0xAA}, ExtensionProfileTwoByte},
+		}
+		for name, testCase := range cases {
+			t.Run(name, func(t *testing.T) {
+				header := Header{Version: 2}
+				assert.NoError(t, header.SetExtension(testCase.id, testCase.payload))
+				assert.Equal(t, testCase.profile, header.ExtensionProfile)
+
+				raw, err := header.Marshal()
+				assert.NoError(t, err)
+
+				var parsed Header
+				_, err = parsed.Unmarshal(raw)
+				assert.NoError(t, err)
+				assert.Equal(t, []uint8{testCase.id}, parsed.GetExtensionIDs())
+				assert.Len(t, parsed.GetExtension(testCase.id), len(testCase.payload))
+			})
+		}
+	})
+
+	t.Run("rejects extensions that cannot be encoded", func(t *testing.T) {
+		header := Header{Version: 2}
+		assert.Error(t, header.SetExtension(1, make([]byte, 256)))
+		assert.Error(t, header.SetExtension(0, []byte{0xAA}))
+
+		// The failed calls must not leave a partial extension behind.
+		assert.False(t, header.Extension)
+		assert.Zero(t, header.ExtensionProfile)
+		assert.Empty(t, header.Extensions)
+	})
+}