When testing NaN against ow.number, the following error message is emitted:

ArgumentError: Expected argument to be of type `number` but received type `number`
at ow (node_modules/ow/dist/index.js:29:28) 
{
  validationErrors: Map(1) {
    'number' => Set(1) {
      'Expected argument to be of type `number` but received type `number`'
    }
  }
}

This behaviour can be verified by running this two line snippet in the Node REPL:

const { default: ow } = require('ow')
ow(NaN, ow.number)

I think that this error message would be quite confusing for those who encounter it. I see two possible ways to solve this problem:

• Change the error message in the NaN case to be something along the lines of: Expected argument to be of type `number`, but received `NaN`.
• Add specific ow.number.nan and ow.number.not.nan checks.