Fix doc tests

[Yijia-Chen]

Fix #231

[codecov-io]

Codecov Report

Merging #248 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #248   +/-   ##
=======================================
  Coverage   90.02%   90.02%           
=======================================
  Files           5        5           
  Lines         441      441           
=======================================
  Hits          397      397           
  Misses         44       44

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update da5ea3a...8f6fc44. Read the comment docs.