diff --git a/vyper/signatures/function_signature.py b/vyper/signatures/function_signature.py
index 8462aa75f8..47a35af722 100644
--- a/vyper/signatures/function_signature.py
+++ b/vyper/signatures/function_signature.py
@@ -168,7 +168,8 @@ def from_definition(cls,
 
         # Validate default values.
         for default_value in getattr(code.args, 'defaults', []):
-            if not isinstance(default_value, (ast.Num, ast.Str, ast.Bytes, ast.List)):
+            allowed_types = (ast.Num, ast.Str, ast.Bytes, ast.List, ast.NameConstant)
+            if not isinstance(default_value, allowed_types):
                 raise FunctionDeclarationException("Default parameter values have to be literals.")
 
         # Determine the arguments, expects something of the form def foo(arg1: