diff --git a/scapy/layers/ntp.py b/scapy/layers/ntp.py
index 51eb4b1002b..c3d2cc111a9 100644
--- a/scapy/layers/ntp.py
+++ b/scapy/layers/ntp.py
@@ -143,25 +143,25 @@ def i2m(self, pkt, val):
 
 # RFC 5905 / Section 7.3
 _reference_identifiers = {
-    "GOES": "Geosynchronous Orbit Environment Satellite",
-    "GPS ": "Global Position System",
-    "GAL ": "Galileo Positioning System",
-    "PPS ": "Generic pulse-per-second",
-    "IRIG": "Inter-Range Instrumentation Group",
-    "WWVB": "LF Radio WWVB Ft. Collins, CO 60 kHz",
-    "DCF ": "LF Radio DCF77 Mainflingen, DE 77.5 kHz",
-    "HBG ": "LF Radio HBG Prangins, HB 75 kHz",
-    "MSF ": "LF Radio MSF Anthorn, UK 60 kHz",
-    "JJY ": "LF Radio JJY Fukushima, JP 40 kHz, Saga, JP 60 kHz",
-    "LORC": "MF Radio LORAN C station, 100 kHz",
-    "TDF ": "MF Radio Allouis, FR 162 kHz",
-    "CHU ": "HF Radio CHU Ottawa, Ontario",
-    "WWV ": "HF Radio WWV Ft. Collins, CO",
-    "WWVH": "HF Radio WWVH Kauai, HI",
-    "NIST": "NIST telephone modem",
-    "ACTS": "NIST telephone modem",
-    "USNO": "USNO telephone modem",
-    "PTB ": "European telephone modem",
+    b"GOES": "Geosynchronous Orbit Environment Satellite",
+    b"GPS ": "Global Position System",
+    b"GAL ": "Galileo Positioning System",
+    b"PPS ": "Generic pulse-per-second",
+    b"IRIG": "Inter-Range Instrumentation Group",
+    b"WWVB": "LF Radio WWVB Ft. Collins, CO 60 kHz",
+    b"DCF ": "LF Radio DCF77 Mainflingen, DE 77.5 kHz",
+    b"HBG ": "LF Radio HBG Prangins, HB 75 kHz",
+    b"MSF ": "LF Radio MSF Anthorn, UK 60 kHz",
+    b"JJY ": "LF Radio JJY Fukushima, JP 40 kHz, Saga, JP 60 kHz",
+    b"LORC": "MF Radio LORAN C station, 100 kHz",
+    b"TDF ": "MF Radio Allouis, FR 162 kHz",
+    b"CHU ": "HF Radio CHU Ottawa, Ontario",
+    b"WWV ": "HF Radio WWV Ft. Collins, CO",
+    b"WWVH": "HF Radio WWVH Kauai, HI",
+    b"NIST": "NIST telephone modem",
+    b"ACTS": "NIST telephone modem",
+    b"USNO": "USNO telephone modem",
+    b"PTB ": "European telephone modem",
 }