Fix autocompletion panic

[tdakkota]

Fixes #178.

[codecov-io]

Codecov Report

Merging #251 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
- Coverage   61.35%   61.32%   -0.04%     
==========================================
  Files          68       68              
  Lines        6345     6345              
==========================================
- Hits         3893     3891       -2     
- Misses       1936     1938       +2     
  Partials      516      516              

Impacted Files	Coverage Δ
engine/memoryengine/store.go	91.96% <0.00%> (-1.79%)	⬇️

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[asdine]

Thanks!