diff --git a/changelog.d/6849.bugfix b/changelog.d/6849.bugfix
new file mode 100644
index 0000000000..d928a26ec6
--- /dev/null
+++ b/changelog.d/6849.bugfix
@@ -0,0 +1 @@
+Fix Synapse refusing to start if `federation_certificate_verification_whitelist` option is blank.
diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index 2e9e478a2a..2514b0713d 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -109,6 +109,8 @@ class TlsConfig(Config):
         fed_whitelist_entries = config.get(
             "federation_certificate_verification_whitelist", []
         )
+        if fed_whitelist_entries is None:
+            fed_whitelist_entries = []
 
         # Support globs (*) in whitelist values
         self.federation_certificate_verification_whitelist = []  # type: List[str]