diff --git a/pylidc/Contour.py b/pylidc/Contour.py
index 24aa396..6aae6d4 100644
--- a/pylidc/Contour.py
+++ b/pylidc/Contour.py
@@ -108,7 +108,7 @@ def to_matrix(self, include_k=True):
         else:
             k  = np.ones(ij.shape[0])*self.image_k_position
             zs = self.annotation.contour_slice_zvals
-            return np.c_[ij, k].astype(int)
+            return np.c_[ij, k].astype(np.int64)
     
 Annotation.contours = relationship('Contour',
                                    order_by=Contour.id,