fix a bug in checking wrong object's type

[imShZh]

The variable guard is always a list object. We should check leaf guard(i.e., variable x)'s type here.

[youkaichao]

is this still necessary after #67 ?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.88%. Comparing base (cbfbc00) to head (2b0a3e6).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   90.82%   90.88%   +0.06%     
==========================================
  Files          10       10              
  Lines        1547     1547              
==========================================
+ Hits         1405     1406       +1     
+ Misses        142      141       -1     

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[imShZh]

is this still necessary after #67 ?

Yes, the other PR does not introduce this modification, as it may not what you want.

PTAL~

[youkaichao]

this is great, thank for fixing it!