diff --git a/pyhocon/config_parser.py b/pyhocon/config_parser.py
index 7ffdb88..a504697 100644
--- a/pyhocon/config_parser.py
+++ b/pyhocon/config_parser.py
@@ -38,11 +38,11 @@ def fixed_get_attr(self, item):
 use_urllib2 = False
 try:
     # For Python 3.0 and later
-    from urllib.request import urlopen
     from urllib.error import HTTPError, URLError
+    from urllib.request import urlopen
 except ImportError:  # pragma: no cover
     # Fall back to Python 2's urllib2
-    from urllib2 import urlopen, HTTPError, URLError
+    from urllib2 import HTTPError, URLError, urlopen
 
     use_urllib2 = True
 try:
@@ -643,9 +643,11 @@ def resolve_substitutions(cls, config, accept_unresolved=False):
                         continue
 
                     is_optional_resolved, resolved_value = cls._resolve_variable(config, substitution)
-                    if isinstance(resolved_value, ConfigValues) and overridden_value and not isinstance(
-                            overridden_value, ConfigValues):
-                        unresolved, _, _ = cls._do_substitute(substitution, overridden_value, is_optional_resolved)
+                    if isinstance(resolved_value, ConfigValues) :
+                        value_to_be_substitute = resolved_value
+                        if overridden_value and not isinstance(overridden_value, ConfigValues):
+                                value_to_be_substitute = overridden_value
+                        unresolved, _, _ = cls._do_substitute(substitution, value_to_be_substitute, is_optional_resolved)
                         any_unresolved = unresolved or any_unresolved
                         if not unresolved and substitution in substitutions:
                             substitutions.remove(substitution)
@@ -656,6 +658,7 @@ def resolve_substitutions(cls, config, accept_unresolved=False):
                         continue
 
                     cache_values = []
+
                     if isinstance(overridden_value, ConfigValues):
                         cache_values = cache.get(substitution)
                         if cache_values is None:
diff --git a/setup.py b/setup.py
index 2d6b631..7127c77 100755
--- a/setup.py
+++ b/setup.py
@@ -26,7 +26,7 @@ def run_tests(self):
 
 setup(
     name='pyhocon',
-    version='0.3.59',
+    version='0.3.61',
     description='HOCON parser for Python',
     long_description='pyhocon is a HOCON parser for Python. Additionally we provide a tool (pyhocon) to convert any HOCON '
                      'content into json, yaml and properties format.',
diff --git a/tests/test_config_parser.py b/tests/test_config_parser.py
index 34e78cd..e81fdcd 100644
--- a/tests/test_config_parser.py
+++ b/tests/test_config_parser.py
@@ -14,10 +14,12 @@
     # Python 2
     from urllib import pathname2url
 
-from pyparsing import ParseBaseException, ParseException, ParseSyntaxException
 import mock
 import pytest
-from pyhocon import (ConfigFactory, ConfigParser, ConfigSubstitutionException, ConfigTree, HOCONConverter)
+from pyparsing import ParseBaseException, ParseException, ParseSyntaxException
+
+from pyhocon import (ConfigFactory, ConfigParser, ConfigSubstitutionException,
+                     ConfigTree, HOCONConverter)
 from pyhocon.exceptions import (ConfigException, ConfigMissingException,
                                 ConfigWrongTypeException)
 
@@ -1486,7 +1488,6 @@ def test_include_dict(self):
                 """.format(tmp_file=incl_name)
             )
             assert config3['a'] == expected_res
-        
         finally:
             os.remove(incl_name)
 
@@ -1747,6 +1748,21 @@ def test_cascade_optional_substitution(self):
             'retries_msg': 'You have 3 retries'
         }
 
+    def test_override_optional_substitution(self):
+        config = ConfigFactory.parse_string(
+            """
+              a = 3
+              test = ${a}
+              test = ${?b}
+              result = ${test}
+            """)
+        assert config == {
+            'a' : 3,
+            'test': 3,
+            'result': 3
+        }
+
+
     def test_substitution_cycle(self):
         with pytest.raises(ConfigSubstitutionException):
             ConfigFactory.parse_string(