diff --git a/xblock/core.py b/xblock/core.py
index 7ff362475..d3ca513d9 100644
--- a/xblock/core.py
+++ b/xblock/core.py
@@ -799,6 +799,7 @@ def __init__(
         """
         if scope_ids is UNSET:
             raise TypeError('scope_ids are required')
+        scope_ids.validate_types()
 
         # A cache of the parent block, retrieved from .parent
         self._parent_block = None
diff --git a/xblock/fields.py b/xblock/fields.py
index 9f814ce06..69c11973e 100644
--- a/xblock/fields.py
+++ b/xblock/fields.py
@@ -3,8 +3,9 @@
 **scopes** to associate each field with particular sets of blocks and users.
 The hosting runtime application decides what actual storage mechanism to use
 for each scope.
-
 """
+from __future__ import annotations
+
 from collections import namedtuple
 import copy
 import datetime
@@ -15,6 +16,8 @@
 import traceback
 import warnings
 
+from opaque_keys.edx.keys import UsageKey, DefinitionKey
+
 import dateutil.parser
 from lxml import etree
 import pytz
@@ -242,6 +245,27 @@ class ScopeIds(namedtuple('ScopeIds', 'user_id block_type def_id usage_id')):
     """
     __slots__ = ()
 
+    def validate_types(self):
+        """
+        Raise an AssertionError if any of the ids are an unexpected type.
+
+        Originally, these fields were all freely-typed; but in practice,
+        edx-platform's XBlock runtime would fail if the ids did not match the
+        types below. In order to make the XBlock library reflect the
+        edx-platform reality and improve type-safety, we've decided to actually
+        enforce the types here, per:
+        https://github.com/openedx/XBlock/issues/708
+        """
+        if self.user_id is not None:
+            if not isinstance(self.user_id, (int, str)):
+                raise TypeError(f"got {self.user_id=}; should be an int, str, or None")
+        if not isinstance(self.block_type, str):
+            raise TypeError(f"got {self.block_type=}; should be a str")
+        if not isinstance(self.def_id, DefinitionKey):
+            raise TypeError(f"got {self.def_id=}; should be a DefinitionKey")
+        if not isinstance(self.usage_id, UsageKey):
+            raise TypeError(f"got {self.usage_id=}; should be a UsageKey")
+
 
 # Define special reference that can be used as a field's default in field
 # definition to signal that the field should default to a unique string value