diff --git a/pymc3/model_graph.py b/pymc3/model_graph.py
index f181554387..e68a28cb17 100644
--- a/pymc3/model_graph.py
+++ b/pymc3/model_graph.py
@@ -1,9 +1,22 @@
-from theano.gof.graph import inputs
+import itertools
+
+from theano.gof.graph import ancestors
 
 from .util import get_default_varnames
 import pymc3 as pm
 
 
+def powerset(iterable):
+    """All *nonempty* subsets of an iterable.
+
+    From itertools docs.
+
+    powerset([1,2,3]) --> (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)
+    """
+    s = list(iterable)
+    return itertools.chain.from_iterable(itertools.combinations(s, r) for r in range(1, len(s)+1))
+
+
 class ModelGraph(object):
     def __init__(self, model):
         self.model = model
@@ -21,30 +34,29 @@ def get_deterministics(self, var):
                 deterministics.append(v)
         return deterministics
 
-    def _inputs(self, var, func, blockers=None):
-        """Get inputs to a function that are also named PyMC3 variables"""
-        return set([j for j in inputs([func], blockers=blockers) if j in self.var_list and j != var])
+    def _ancestors(self, var, func, blockers=None):
+        """Get ancestors of a function that are also named PyMC3 variables"""
+        return set([j for j in ancestors([func], blockers=blockers) if j in self.var_list and j != var])
 
-    def _get_inputs(self, var, func):
-        """Get all inputs to a function, doing some accounting for deterministics
+    def _get_ancestors(self, var, func):
+        """Get all ancestors of a function, doing some accounting for deterministics
 
-        Specifically, if a deterministic is an input, theano.gof.graph.inputs will
+        Specifically, if a deterministic is an input, theano.gof.graph.ancestors will
         return only the inputs *to the deterministic*.  However, if we pass in the
         deterministic as a blocker, it will skip those nodes.
         """
         deterministics = self.get_deterministics(var)
-        upstream = self._inputs(var, func)
-        parents = self._inputs(var, func, blockers=deterministics)
-        if parents != upstream:
-            det_map = {}
-            for d in deterministics:
-                d_set = {j for j in inputs([func], blockers=[d])}
-                if upstream - d_set:
-                    det_map[d] = d_set
-            for d, d_set in det_map.items():
-                if all(d_set.issubset(other) for other in det_map.values()):
-                    parents.add(d)
-        return parents
+        upstream = self._ancestors(var, func)
+
+        # Usual case
+        if upstream == self._ancestors(var, func, blockers=upstream):
+            return upstream
+        else: # deterministic accounting
+            for d in powerset(upstream):
+                blocked = self._ancestors(var, func, blockers=d)
+                if set(d) == blocked:
+                    return d
+        raise RuntimeError('Could not traverse graph. Consider raising an issue with developers.')
 
     def _filter_parents(self, var, parents):
         """Get direct parents of a var, as strings"""
@@ -70,7 +82,7 @@ def get_parents(self, var):
         else:
             func = var
 
-        parents = self._get_inputs(var, func)
+        parents = self._get_ancestors(var, func)
         return self._filter_parents(var, parents)
 
     def make_compute_graph(self):