Doesn't look like it likes xkcd hand lettering or the font that approximates it. I had better luck with a this...
curl https://ocr.a9t9.com/api/Parse/Image --data "apikey=helloworld&url=http://www.uky.edu/Providers/ScannedText/page1s.jpeg"
{"ParsedResults":[{"FileParseExitCode":1,"ParsedText":"In 1830 there were but twenty-three \r\nmiles of railroad in operation in the \r\nUnited States, and in that year Ken- \r\ntucky took the initial step in the work \r\nwest of fhe Alleghanies. An Act to \r\nincorporate the Lexington & Ohio \r\nRailway Company was approved by \r\nGov. Metcalf, Jarinary 27, 1830.. It \r\nprovided for the construction and reÔÇó \r\n","ErrorMessage":"","ErrorDetails":""}],"OCRExitCode":1,"IsErroredOnProcessing":false,"ErrorMessage":null,"ErrorDetails":null}
curl https://ocr.a9t9.com/api/Parse/Image --data "apikey=helloworld&url=http://imgs.xkcd.com/comics/bells_theorem.png" { "ParsedResults": [ { "FileParseExitCode": 1, "ParsedText": "T-as IS CALLED I THEOREM. IT LAS FIRST\u2014 t: 1 wostcop n, FASTER- FIRM-LIGHT com)MCBT10N IS E6SlBLE! EL's rta\u201eNDERSTRtDlNGS ELLS THEX\u00dcI I-IPPPEN VIOLATE LOCALITY", "ErrorMessage": "", "ErrorDetails": "" } ], "OCRExitCode": 1, "IsErroredOnProcessing": false, "ErrorMessage": null, "ErrorDetails": null }