diff --git a/apiclient/schema.py b/apiclient/schema.py
index ddcd670..cfed7de 100644
--- a/apiclient/schema.py
+++ b/apiclient/schema.py
@@ -244,9 +244,13 @@
     if stype == 'object':
       self.emitEnd('{', schema.get('description', ''))
       self.indent()
-      for pname, pschema in schema.get('properties', {}).iteritems():
-        self.emitBegin('"%s": ' % pname)
-        self._to_str_impl(pschema)
+      if 'properties' in schema:
+        for pname, pschema in schema.get('properties', {}).iteritems():
+          self.emitBegin('"%s": ' % pname)
+          self._to_str_impl(pschema)
+      elif 'additionalProperties' in schema:
+        self.emitBegin('"a_key": ')
+        self._to_str_impl(schema['additionalProperties'])
       self.undent()
       self.emit('},')
     elif '$ref' in schema: