diff --git a/lms/djangoapps/course_api/blocks/utils.py b/lms/djangoapps/course_api/blocks/utils.py
index 0af24b951ade..6f371624b7df 100644
--- a/lms/djangoapps/course_api/blocks/utils.py
+++ b/lms/djangoapps/course_api/blocks/utils.py
@@ -11,47 +11,55 @@
 
 def filter_discussion_xblocks_from_response(response, course_key):
     """
-    Removes discussion xblocks if discussion provider is openedx
+    Removes discussion xblocks if discussion provider is openedx.
     """
     configuration = DiscussionsConfiguration.get(context_key=course_key)
     provider = configuration.provider_type
-    if provider == Provider.OPEN_EDX:
-        # Finding ids of discussion xblocks
-        if isinstance(response.data, ReturnList):
-            discussion_xblocks = [
-                value.get('id') for value in response.data if value.get('type') == 'discussion'
-            ]
-        else:
-            discussion_xblocks = [
-                key for key, value in response.data.get('blocks', {}).items()
-                if value.get('type') == 'discussion'
-            ]
-        # Filtering discussion xblocks keys from blocks
-        if isinstance(response.data, ReturnList):
-            filtered_blocks = {
-                value.get('id'): value
-                for value in response.data
-                if value.get('type') != 'discussion'
-            }
-        else:
-            filtered_blocks = {
-                key: value
-                for key, value in response.data.get('blocks', {}).items()
-                if value.get('type') != 'discussion'
-            }
-        # Removing reference of discussion xblocks from unit
-        # These references needs to be removed because they no longer exist
-        for _, block_data in filtered_blocks.items():
-            for key in ['descendants', 'children']:
-                descendants = block_data.get(key, [])
-                if descendants:
-                    descendants = [
-                        descendant for descendant in descendants
-                        if descendant not in discussion_xblocks
-                    ]
-                    block_data[key] = descendants
-        if isinstance(response.data, ReturnList):
-            response.data = filtered_blocks
-        else:
-            response.data['blocks'] = filtered_blocks
+
+    if provider != Provider.OPEN_EDX:
+        return response
+
+    is_list_response = isinstance(response.data, ReturnList)
+
+    # Find discussion xblock IDs
+    if is_list_response:
+        discussion_xblocks = [
+            block.get('id') for block in response.data
+            if block.get('type') == 'discussion'
+        ]
+    else:
+        discussion_xblocks = [
+            key for key, value in response.data.get('blocks', {}).items()
+            if value.get('type') == 'discussion'
+        ]
+
+    # Filter out discussion blocks
+    if is_list_response:
+        filtered_blocks = [
+            block for block in response.data
+            if block.get('type') != 'discussion'
+        ]
+    else:
+        filtered_blocks = {
+            key: value for key, value in response.data.get('blocks', {}).items()
+            if value.get('type') != 'discussion'
+        }
+
+    # Remove references to discussion xblocks
+    # These references needs to be removed because they no longer exist
+    blocks_iterable = filtered_blocks if is_list_response else filtered_blocks.values()
+    for block_data in blocks_iterable:
+        for key in ['descendants', 'children']:
+            if key in block_data:
+                block_data[key] = [
+                    descendant for descendant in block_data[key]
+                    if descendant not in discussion_xblocks
+                ]
+
+    # Update response
+    if is_list_response:
+        response.data = ReturnList(filtered_blocks, serializer=None)
+    else:
+        response.data['blocks'] = filtered_blocks
+
     return response
diff --git a/lms/djangoapps/course_api/blocks/views.py b/lms/djangoapps/course_api/blocks/views.py
index 0697e9321989..96679a562957 100644
--- a/lms/djangoapps/course_api/blocks/views.py
+++ b/lms/djangoapps/course_api/blocks/views.py
@@ -330,12 +330,14 @@ def list(self, request, hide_access_denials=False):  # pylint: disable=arguments
 
                 if course_block.get('type') == 'course':
                     root = course_block['id']
+                else:
+                    root = str(course_usage_key)
         else:
             root = response.data['root']
             course_blocks = response.data['blocks']
 
         if not root:
-            raise ValueError(f"Unable to find course block in {course_key_string}")
+            raise ValidationError(f"Unable to find course block in '{course_key_string}'")
 
         recurse_mark_complete(root, course_blocks)
         return response