I wouldn't necessarily use acetonitrile as the solvent. At least in my case (using p-nitrobenzaldoxime), I was using acetonitrile as reactant and solvent, to form the 3-aryl-5-methyl-1,2,4-oxadiazole. I'd think that water may potentially rehydrate the nitrile oxide back to a nitro compound (isn't that another way to form the nitrile oxide, dehydration of a nitro? Is the nitro group you have attached to a tertiary carbon?)