diff --git a/datasette/default_magic_parameters.py b/datasette/default_magic_parameters.py
index b788fee8..0f8f397e 100644
--- a/datasette/default_magic_parameters.py
+++ b/datasette/default_magic_parameters.py
@@ -8,7 +8,7 @@ import time
 def header(key, request):
     key = key.replace("_", "-").encode("utf-8")
     headers_dict = dict(request.scope["headers"])
-    return headers_dict[key].decode("utf-8")
+    return headers_dict.get(key, b"").decode("utf-8")
 
 
 def actor(key, request):
diff --git a/tests/test_canned_queries.py b/tests/test_canned_queries.py
index 2e064db1..c0219cb1 100644
--- a/tests/test_canned_queries.py
+++ b/tests/test_canned_queries.py
@@ -186,6 +186,7 @@ def magic_parameters_client():
     [
         ("_actor_id", "root"),
         ("_header_host", "localhost"),
+        ("_header_not_a_thing", ""),
         ("_cookie_foo", "bar"),
         ("_now_epoch", r"^\d+$"),
         ("_now_date_utc", r"^\d{4}-\d{2}-\d{2}$"),