XML parsing error: whitespace expected

In one of the XML imports I had created for FileMaker I was continually receiving the following error:

I couldn’t figure out what it meant? What was a XML parsing error: whitespace expected?  I had rewritten my xsl stylesheet sever times with no success.  Finally, on a hunch i removed all spaces from my xsl stylesheet name.  Wola!  no more error. (ex. ICD10CMChapterInfo.xsl  vs. ICD10 CM Chapter Info.xsl)