db.foo.drop();

db.foo.insert({a:'a', b:['b','c']});

db.runCommand({analyze: "foo", key: 'a', numberBuckets: 1000});
db.runCommand({analyze: "foo", key: 'b', numberBuckets: 1000});

db.adminCommand({setParameter: 1, planRankerMode: "samplingCE"});

db.foo.createIndex({a:-1});
# This returns 1 , correctly.
db.foo.find({a:'a'}).explain().queryPlanner.winningPlan;
db.foo.dropIndex('a_-1');

db.foo.createIndex({a:1, b:-1});
# This returns zero, incorrectly
db.foo.find({a:'a'}).explain().queryPlanner.winningPlan;

{
  isCached: false,
  stage: 'FETCH',
  costEstimate: 0.0074987,
  cardinalityEstimate: 0,
  estimatesMetadata: { ceSource: 'Sampling' },
  inputStage: {
    stage: 'IXSCAN',
    costEstimate: 0.000009999999999999999,
    cardinalityEstimate: 0, <<<=== wrong
    numKeysEstimate: 0, <<<=== wrong
    estimatesMetadata: { ceSource: 'Sampling' },
    keyPattern: { a: 1, b: -1 },
    indexName: 'a_1_b_-1',
    isMultiKey: true,
    multiKeyPaths: { a: [], b: [ 'b' ] },
    isUnique: false,
    isSparse: false,
    isPartial: false,
    indexVersion: 2,
    direction: 'forward',
    indexBounds: { a: [ '["a", "a"]' ], b: [ '[MaxKey, MinKey]' ] }
  }
}

db.foo.dropIndex('a_1_b_-1');