diff --git a/Lib/ast.py b/Lib/ast.py
index 157a833..0c88bcf 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -59,11 +59,12 @@
         node_or_string = parse(node_or_string, mode='eval')
     if isinstance(node_or_string, Expression):
         node_or_string = node_or_string.body
+    def _raise_malformed_node(node):
+        raise ValueError(f'malformed node or string: {node!r}')
     def _convert_num(node):
-        if isinstance(node, Constant):
-            if type(node.value) in (int, float, complex):
-                return node.value
-        raise ValueError('malformed node or string: ' + repr(node))
+        if not isinstance(node, Constant) or type(node.value) not in (int, float, complex):
+            _raise_malformed_node(node)
+        return node.value
     def _convert_signed_num(node):
         if isinstance(node, UnaryOp) and isinstance(node.op, (UAdd, USub)):
             operand = _convert_num(node.operand)
@@ -82,6 +83,8 @@
         elif isinstance(node, Set):
             return set(map(_convert, node.elts))
         elif isinstance(node, Dict):
+            if len(node.keys) != len(node.values):
+                _raise_malformed_node(node)
             return dict(zip(map(_convert, node.keys),
                             map(_convert, node.values)))
         elif isinstance(node, BinOp) and isinstance(node.op, (Add, Sub)):