diff --git a/xblock/core.py b/xblock/core.py
index 81d0fbf36..c9f5d0c8f 100644
--- a/xblock/core.py
+++ b/xblock/core.py
@@ -355,19 +355,20 @@ def _set_field_if_present(cls, block, name, value, attrs):
         else:
             logging.warning("%s does not contain field %s", type(block), name)
 
-    def __init__(self, runtime, scope_ids, field_data=None, **kwargs):
+    def __init__(self, scope_ids, field_data=None, *, runtime, **kwargs):
         """
         Arguments:
 
-            runtime (:class:`.Runtime`): Use it to access the environment.
-                It is available in XBlock code as ``self.runtime``.
-
             scope_ids (:class:`.ScopeIds`): Identifiers needed to resolve
                 scopes.
 
             field_data (:class:`.FieldData`): Interface used by XBlock-likes'
                 fields to access their data from wherever it is persisted.
                 DEPRECATED--supply a field-data Runtime service instead.
+
+            runtime (:class:`.Runtime`): Use it to access the environment.
+                It is available in XBlock code as ``self.runtime``.
+
         """
         self.runtime = runtime