diff --git a/wfl/configset.py b/wfl/configset.py
index 2bad8242..ede23f7c 100644
--- a/wfl/configset.py
+++ b/wfl/configset.py
@@ -144,6 +144,18 @@ def __iter__(self):
         if self.items is None:
             return
 
+        # special handling for ConfigSet generated by groups() iterator, which 
+        # sets self._cur_at, self._file_loc, and self._open_reader
+        if self._file_loc is not None and len(self._file_loc) > 0 and self._cur_at[0] is not None:
+            assert self._open_reader is not None
+            while self._cur_at[0].info["_ConfigSet_loc"].startswith(self._file_loc):
+                yield self._cur_at[0]
+                try:
+                    self._cur_at[0] = next(self._open_reader)
+                except StopIteration:
+                    break
+            return
+
         if isinstance(self.items, Path):
             # yield Atoms from one file
             ## print("DEBUG __iter__ one file", self.items)