diff --git a/pyhocon/config_parser.py b/pyhocon/config_parser.py
index cc1c1de..39b19a3 100644
--- a/pyhocon/config_parser.py
+++ b/pyhocon/config_parser.py
@@ -467,17 +467,18 @@ def _resolve_variable(cls, config, substitution):
     @classmethod
     def _fixup_self_references(cls, config, accept_unresolved=False):
         if isinstance(config, ConfigTree) and config.root:
-            for key in config:  # Traverse history of element
-                history = config.history[key]
+            for key in list(config.keys()):  # Use list to avoid mutation error during iteration
+                history = config.history.get(key)
+                if not history:
+                    continue
                 previous_item = history[0]
                 for current_item in history[1:]:
                     for substitution in cls._find_substitutions(current_item):
                         prop_path = ConfigTree.parse_key(substitution.variable)
                         if len(prop_path) > 1 and config.get(substitution.variable, None) is not None:
-                            continue  # If value is present in latest version, don't do anything
+                            continue
                         if prop_path[0] == key:
                             if isinstance(previous_item, ConfigValues) and not accept_unresolved:
-                                # We hit a dead end, we cannot evaluate
                                 raise ConfigSubstitutionException(
                                     "Property {variable} cannot be substituted. Check for cycles.".format(
                                         variable=substitution.variable
@@ -493,13 +494,13 @@ def _fixup_self_references(cls, config, accept_unresolved=False):
                     for substitution in cls._find_substitutions(previous_item):
                         prop_path = ConfigTree.parse_key(substitution.variable)
                         if len(prop_path) > 1 and config.get(substitution.variable, None) is not None:
-                            continue  # If value is present in latest version, don't do anything
+                            continue
                         if prop_path[0] == key:
                             value = os.environ.get(key)
                             if value is not None:
                                 cls._do_substitute(substitution, value)
                                 continue
-                            if substitution.optional:  # special case, when self optional referencing without existing
+                            if substitution.optional:
                                 cls._do_substitute(substitution, None)
 
     # traverse config to find all the substitutions