Error in knnsearch when using Ionosphere dataset

Hi I am receiving the following error when trying to run the knnsearch function with the standard Ionosphere dataset. Could someone please help?

Error using ExhaustiveSearcher/setDistParameter (line 110) The Cov argument must be symmetric and positive definite.

Error in NeighborSearcher/set.DistParameter (line 71) this = setDistParameter(this, distPara);

Error in ExhaustiveSearcher (line 274) obj.DistParameter = nancov(X);

Error in createns (line 174) O = ExhaustiveSearcher(X,'distance', distMetric,...

Error in knnsearch (line 140) O=createns(X,args{:},'nsmethod', nsmethod,'bucketSize',bSize);

Error in MyPgm (line 37) [knn] = knnsearch(dataset, dataset, 'k', K, 'distance',distMeasure);

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Thanks in advance Deepa

[Walter Roberson]

What distMeasure did you pass in?