diff --git a/pymc/utils.py b/pymc/utils.py
index 52baf955..6628931c 100644
--- a/pymc/utils.py
+++ b/pymc/utils.py
@@ -45,9 +45,8 @@
 PY3 = sys.version_info >= (3, 3)
 
 def get_signature_py3(func):
-    sig = inspect.signature(func)
-    defaults = tuple(p.default for p in sig.parameters.values() if p.default is not inspect._empty)
-    args = [k for k in sig.parameters.keys() if k not in ('args', 'kwds')]
+    (args, varargs, varkw, defaults, kwonlyargs, kwonlydefaults,
+        annotations) = inspect.getfullargspec(func)
     return args, defaults
     
 def get_signature_py2(func):