diff --git a/CHANGELOG.md b/CHANGELOG.md
index d8f3064d4..fedcd1f44 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,11 @@
 -
 -
 
+## HDMF 2.5.7 (June 4, 2021)
+
+### Bug fix
+- Fix generation of extension classes that extend MultiContainerInterface and use a custom _fieldsname. @rly (#626)
+
 ## HDMF 2.5.6 (May 19, 2021)
 
 ### Bug fix
diff --git a/src/hdmf/build/__init__.py b/src/hdmf/build/__init__.py
index 5f467d916..ea5d21152 100644
--- a/src/hdmf/build/__init__.py
+++ b/src/hdmf/build/__init__.py
@@ -1,5 +1,5 @@
 from .builders import Builder, DatasetBuilder, GroupBuilder, LinkBuilder, ReferenceBuilder, RegionBuilder
-from .classgenerator import CustomClassGenerator
+from .classgenerator import CustomClassGenerator, MCIClassGenerator
 from .errors import (BuildError, OrphanContainerBuildError, ReferenceTargetNotBuiltError, ContainerConfigurationError,
                      ConstructError)
 from .manager import BuildManager, TypeMap
diff --git a/src/hdmf/build/classgenerator.py b/src/hdmf/build/classgenerator.py
index edd95a6e7..db4f326d3 100644
--- a/src/hdmf/build/classgenerator.py
+++ b/src/hdmf/build/classgenerator.py
@@ -268,9 +268,10 @@ def post_process(cls, classdict, bases, docval_args, spec):
         :param spec: The spec for the container class to generate.
         """
         # convert classdict['__fields__'] from list to tuple if present
-        fields = classdict.get(bases[0]._fieldsname)
-        if fields is not None:
-            classdict[bases[0]._fieldsname] = tuple(fields)
+        for b in bases:
+            fields = classdict.get(b._fieldsname)
+            if fields is not None and not isinstance(fields, tuple):
+                classdict[b._fieldsname] = tuple(fields)
 
         # if spec provides a fixed name for this type, remove the 'name' arg from docval_args so that values cannot
         # be passed for a name positional or keyword arg