当我使用python-reportlab创建pdf文档时，有时会抛出异常：ValueError: format not resolved talk.google.com，我想知道为什么会出现这种情况，以及如何解决它，完整的错误堆栈如下所示：
File "/usr/lib64/python2.7/threading.py", line 552, in __bootstrap_inner
self.run()
File "/usr/lib64/python2.7/threading.py", line 505, in run
self.__target(*self.__args, **self.__kwargs)
File "/usr/lib/python2.7/site-packages/tweets2pdf/tweets2pdf.py", line 42,
in generate_thread
tpdoc.dump()
File "/usr/lib/python2.7/site-packages/tweets2pdf/pdfgen.py", line 609, in
dump
self.pdfdoc.build(self.elements, onFirstPage = self.on_first_page,
onLaterPages = self.on_later_pages)
File "/usr/lib64/python2.7/site-
packages/reportlab/platypus/doctemplate.py", line 1117, in build
BaseDocTemplate.build(self,flowables, canvasmaker=canvasmaker)
File "/usr/lib64/python2.7/site-
packages/reportlab/platypus/doctemplate.py", line 906, in build
self._endBuild()
File "/usr/lib64/python2.7/site-
packages/reportlab/platypus/doctemplate.py", line 848, in _endBuild
if getattr(self,'_doSave',1): self.canv.save()
File "/usr/lib64/python2.7/site-packages/reportlab/pdfgen/canvas.py", line
1123, in save
self._doc.SaveToFile(self._filename, self)
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 235, in SaveToFile
f.write(self.GetPDFData(canvas))
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 257, in GetPDFData
return self.format()
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 417, in format
IOf = IO.format(self)
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 869, in format
fcontent = format(self.content, document, toplevel=1) # yes this is at
top level
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 102, in format
f = element.format(document)
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 1635, in format
return D.format(document)
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 667, in format
L = [(format(PDFName(k),document)+" "+format(dict[k],document)) for k in
keys]
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 102, in format
f = element.format(document)
File "/usr/lib64/python2.7/site-packages/reportlab/pdfbase/pdfdoc.py",
line 1764, in format
if f is None: raise ValueError, "format not resolved %s" % self.name
ValueError: format not resolved talk.google.com