diff --git a/src/hdmf/build/map.py b/src/hdmf/build/map.py
index 2fb4e7487..5558acfa1 100644
--- a/src/hdmf/build/map.py
+++ b/src/hdmf/build/map.py
@@ -667,7 +667,7 @@ def get_attr_spec(self, **kwargs):
     def get_carg_spec(self, **kwargs):
         """ Return the Spec for a given constructor argument """
         carg_name = getargs('carg_name', kwargs)
-        return self.__attr2spec.get(carg_name)
+        return self.__carg2spec.get(carg_name)
 
     @docval({"name": "const_arg", "type": str, "doc": "the name of the constructor argument to map"},
             {"name": "spec", "type": Spec, "doc": "the spec to map the attribute to"})