diff --git a/pymc3/backends/ndarray.py b/pymc3/backends/ndarray.py
index 700722a787..5fc55da334 100644
--- a/pymc3/backends/ndarray.py
+++ b/pymc3/backends/ndarray.py
@@ -50,7 +50,7 @@ def setup(self, draws, chain, sampler_vars=None):
         if self.samples:  # Concatenate new array if chain is already present.
             old_draws = len(self)
             self.draws = old_draws + draws
-            self.draws_idx = old_draws
+            self.draw_idx = old_draws
             for varname, shape in self.var_shapes.items():
                 old_var_samples = self.samples[varname]
                 new_var_samples = np.zeros((draws, ) + shape,