diff --git a/parso/grammar.py b/parso/grammar.py
index e77d83c..caf5001 100644
--- a/parso/grammar.py
+++ b/parso/grammar.py
@@ -186,7 +186,7 @@ class Grammar(object):
         return normalizer.issues
 
     def __repr__(self):
-        nonterminals = self._pgen_grammar._nonterminal_to_dfas.keys()
+        nonterminals = self._pgen_grammar.nonterminal_to_dfas.keys()
         txt = ' '.join(list(nonterminals)[:3]) + ' ...'
         return '<%s:%s>' % (self.__class__.__name__, txt)
 
diff --git a/parso/parser.py b/parso/parser.py
index b710ed5..859e3f8 100644
--- a/parso/parser.py
+++ b/parso/parser.py
@@ -152,9 +152,12 @@ class BaseParser(object):
 
     def convert_node(self, nonterminal, children):
         try:
-            return self.node_map[nonterminal](children)
+            node = self.node_map[nonterminal](children)
         except KeyError:
-            return self.default_node(nonterminal, children)
+            node = self.default_node(nonterminal, children)
+        for c in children:
+            c.parent = node
+        return node
 
     def convert_leaf(self, type_, value, prefix, start_pos):
         try:
diff --git a/parso/python/diff.py b/parso/python/diff.py
index 24bc795..3551ee4 100644
--- a/parso/python/diff.py
+++ b/parso/python/diff.py
@@ -60,7 +60,7 @@ def _assert_valid_graph(node):
         assert node.start_pos == actual, (node.start_pos, actual)
     else:
         for child in children:
-            assert child.parent == node
+            assert child.parent == node, (node, child)
             _assert_valid_graph(child)
 
 
diff --git a/parso/python/parser.py b/parso/python/parser.py
index 550bc4c..e46b618 100644
--- a/parso/python/parser.py
+++ b/parso/python/parser.py
@@ -90,7 +90,7 @@ class Parser(BaseParser):
         strictly bottom-up.
         """
         try:
-            return self.node_map[nonterminal](children)
+            node = self.node_map[nonterminal](children)
         except KeyError:
             if nonterminal == 'suite':
                 # We don't want the INDENT/DEDENT in our parser tree. Those
@@ -104,7 +104,10 @@ class Parser(BaseParser):
             elif nonterminal == 'listmaker':
                 # Same as list_if above.
                 nonterminal = 'testlist_comp'
-            return self.default_node(nonterminal, children)
+            node = self.default_node(nonterminal, children)
+        for c in children:
+            c.parent = node
+        return node
 
     def convert_leaf(self, type, value, prefix, start_pos):
         # print('leaf', repr(value), token.tok_name[type])
@@ -189,7 +192,10 @@ class Parser(BaseParser):
         all_nodes = [node for stack_node in self.stack[start_index:] for node in stack_node.nodes]
 
         if all_nodes:
-            self.stack[start_index - 1].nodes.append(tree.PythonErrorNode(all_nodes))
+            node = tree.PythonErrorNode(all_nodes)
+            for n in all_nodes:
+                n.parent = node
+            self.stack[start_index - 1].nodes.append(node)
 
         self.stack[start_index:] = []
         return bool(all_nodes)
diff --git a/parso/tree.py b/parso/tree.py
index 9e7ab2f..9758a82 100644
--- a/parso/tree.py
+++ b/parso/tree.py
@@ -244,8 +244,6 @@ class BaseNode(NodeOrLeaf):
     type = None
 
     def __init__(self, children):
-        for c in children:
-            c.parent = self
         self.children = children
         """
         A list of :class:`NodeOrLeaf` child nodes.