diff --git a/src/hdmf/backends/hdf5/h5_utils.py b/src/hdmf/backends/hdf5/h5_utils.py
index f5d74dd5e..278735fbc 100644
--- a/src/hdmf/backends/hdf5/h5_utils.py
+++ b/src/hdmf/backends/hdf5/h5_utils.py
@@ -111,6 +111,12 @@ def shape(self):
     def append(self, arg):
         # Get Builder
         builder = self.io.manager.get_builder(arg)
+        if builder is None:
+            raise ValueError(
+                "The container being appended to the dataset has not yet been built. "
+                "Please write the container to the file, then open the modified file, and "
+                "append the read container to the dataset."
+            )
 
         # Get HDF5 Reference
         ref = self.io._create_ref(builder)