diff --git a/synapse/http/servlet.py b/synapse/http/servlet.py
index f0ca7d9ab..fd07bf7b8 100644
--- a/synapse/http/servlet.py
+++ b/synapse/http/servlet.py
@@ -166,7 +166,12 @@ def parse_string_from_args(
         value = args[name][0]
 
         if encoding:
-            value = value.decode(encoding)
+            try:
+                value = value.decode(encoding)
+            except ValueError:
+                raise SynapseError(
+                    400, "Query parameter %r must be %s" % (name, encoding)
+                )
 
         if allowed_values is not None and value not in allowed_values:
             message = "Query parameter %r must be one of [%s]" % (