diff --git a/crates/clawhdf5-format/src/vds.rs b/crates/clawhdf5-format/src/vds.rs
index 23a3a52..f5d5237 100644
--- a/crates/clawhdf5-format/src/vds.rs
+++ b/crates/clawhdf5-format/src/vds.rs
@@ -831,6 +831,16 @@ impl<'a, 'r> Sources<'a, 'r> {
         path: &str,
         datatype: &Datatype,
     ) -> Result<Option<SourceData>, FormatError> {
+        // Variable-length and reference elements are addresses into the file
+        // that holds them (global-heap IDs, object addresses). Copied out of
+        // another file they would be decoded against the virtual dataset's
+        // file and name some other object, so refuse rather than return them.
+        if file != "." && holds_file_addresses(datatype) {
+            return Err(vds_err(format!(
+                "VDS source {path} in {file}: variable-length and reference data \
+                 from another file is not supported"
+            )));
+        }
         let Some(bytes) = self.file(file)? else {
             return Ok(None);
         };
@@ -841,6 +851,21 @@ impl<'a, 'r> Sources<'a, 'r> {
     }
 }
 
+/// Whether elements of `dt` contain addresses into their own file:
+/// variable-length data (global-heap IDs) or references.
+fn holds_file_addresses(dt: &Datatype) -> bool {
+    match dt {
+        Datatype::VariableLength { .. } | Datatype::Reference { .. } => true,
+        Datatype::Compound { members, .. } => {
+            members.iter().any(|m| holds_file_addresses(&m.datatype))
+        }
+        Datatype::Array { base_type, .. } | Datatype::Enumeration { base_type, .. } => {
+            holds_file_addresses(base_type)
+        }
+        _ => false,
+    }
+}
+
 /// An opened source dataset's object header.
 struct OpenSource {
     offset_size: u8,
diff --git a/crates/clawhdf5/tests/vds_interop.rs b/crates/clawhdf5/tests/vds_interop.rs
index 2346a5e..17069b0 100644
--- a/crates/clawhdf5/tests/vds_interop.rs
+++ b/crates/clawhdf5/tests/vds_interop.rs
@@ -290,6 +290,44 @@ expect("nested.h5", "v", "nested")
     assert_matches_libhdf5(dir.path(), "nested.h5", "v", "nested");
 }
 
+/// Variable-length and reference elements are addresses into their own file.
+/// Copied raw from an external source they would be decoded against the
+/// virtual dataset's file and name another object, so they are refused.
+#[test]
+fn vds_external_variable_length_source_is_an_error_not_foreign_addresses() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    generate(
+        dir.path(),
+        r#"
+st = h5py.string_dtype()
+with h5py.File("src.h5", "w") as s:
+    s.create_dataset("names", data=np.array(["alpha", "beta", "gamma"], dtype=object), dtype=st)
+    s.create_dataset("refs", data=[s.ref, s.ref], dtype=h5py.ref_dtype)
+with h5py.File("v.h5", "w", libver="latest") as f:
+    f.create_dataset("pad", data=np.arange(64.0))
+    lay = h5py.VirtualLayout(shape=(3,), dtype=st)
+    lay[:] = h5py.VirtualSource("src.h5", "names", shape=(3,))
+    f.create_virtual_dataset("names", lay)
+    lay = h5py.VirtualLayout(shape=(2,), dtype=h5py.ref_dtype)
+    lay[:] = h5py.VirtualSource("src.h5", "refs", shape=(2,))
+    f.create_virtual_dataset("refs", lay)
+"#,
+    );
+    let f = File::open(dir.path().join("v.h5")).unwrap();
+    for name in ["names", "refs"] {
+        let err = f
+            .dataset(name)
+            .unwrap()
+            .read_selection(&clawhdf5_format::selection::Selection::All)
+            .expect_err("raw addresses from another file must not be returned");
+        assert!(
+            err.to_string().contains("from another file"),
+            "{name}: unexpected error: {err}"
+        );
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Unlimited and printf-style mappings
 // ---------------------------------------------------------------------------