diff --git a/CHANGES.rst b/CHANGES.rst
index c4abc8707..eabf76e3a 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -22,6 +22,10 @@
 - Fix bug that occurred when attempting to open invalid file but Astropy import
   fails while checking for ASDF-in-FITS. [#562]
 
+- Fix bug that caused tree creation to fail when unable to locate a schema file
+  for an unknown tag. This now simply causes a warning, and the offending node
+  is converted to basic Python data structures. [#571]
+
 2.1.0 (2018-09-25)
 ------------------
 
diff --git a/asdf/schema.py b/asdf/schema.py
index 699bb5ce8..d7ac26378 100644
--- a/asdf/schema.py
+++ b/asdf/schema.py
@@ -4,6 +4,7 @@
 import os
 import json
 import datetime
+import warnings
 from numbers import Integral
 from functools import lru_cache
 from collections import OrderedDict
@@ -218,7 +219,12 @@ def iter_errors(self, instance, _schema=None, _seen=set()):
                 if tag is not None:
                     schema_path = self.ctx.resolver(tag)
                     if schema_path != tag:
-                        s = load_schema(schema_path, self.ctx.resolver)
+                        try:
+                            s = load_schema(schema_path, self.ctx.resolver)
+                        except FileNotFoundError:
+                            msg = "Unable to locate schema file for '{}': '{}'"
+                            warnings.warn(msg.format(tag, schema_path))
+                            s = {}
                         if s:
                             with self.resolver.in_scope(schema_path):
                                 for x in super(ASDFValidator, self).iter_errors(instance, s):
diff --git a/asdf/tests/test_schema.py b/asdf/tests/test_schema.py
index 57cb7a8a5..0aa03b0d2 100644
--- a/asdf/tests/test_schema.py
+++ b/asdf/tests/test_schema.py
@@ -730,3 +730,50 @@ def test_custom_validation_with_definitions_bad(tmpdir):
     with pytest.raises(ValidationError):
         with asdf.open(asdf_file, custom_schema=custom_schema_path) as ff:
             pass
+
+
+def test_nonexistent_tag(tmpdir):
+    """
+    This tests the case where a node is tagged with a type that apparently
+    comes from an extension that is known, but the type itself can't be found.
+
+    This could occur when a more recent version of an installed package
+    provides the new type, but an older version of the package is installed.
+    ASDF should still be able to open the file in this case, but it won't be
+    able to restore the type.
+
+    The bug that prompted this test results from attempting to load a schema
+    file that doesn't exist, which is why this test belongs in this file.
+    """
+
+    # This shouldn't ever happen, but it's a useful test case
+    yaml = """
+a: !core/doesnt_exist-1.0.0
+  hello
+    """
+
+    buff = helpers.yaml_to_asdf(yaml)
+    with pytest.warns(None) as w:
+        with asdf.open(buff) as af:
+            assert str(af['a']) == 'hello'
+        # Currently there are 3 warnings since one occurs on each of the
+        # validation passes. It would be good to consolidate these eventually
+        assert len(w) == 3, helpers.display_warnings(w)
+        assert str(w[0].message).startswith("Unable to locate schema file")
+        assert str(w[1].message).startswith("Unable to locate schema file")
+        assert str(w[2].message).startswith(af['a']._tag)
+
+    # This is a more realistic case since we're using an external extension
+    yaml = """
+a: !<tag:nowhere.org:custom/doesnt_exist-1.0.0>
+  hello
+  """
+
+    buff = helpers.yaml_to_asdf(yaml)
+    with pytest.warns(None) as w:
+        with asdf.open(buff, extensions=CustomExtension()) as af:
+            assert str(af['a']) == 'hello'
+        assert len(w) == 3, helpers.display_warnings(w)
+        assert str(w[0].message).startswith("Unable to locate schema file")
+        assert str(w[1].message).startswith("Unable to locate schema file")
+        assert str(w[2].message).startswith(af['a']._tag)