CATiledlayer setContents nil give me a crash

CATiledlayer setContents nil give me a crash

I have a uiview with catiledlayer,I want to reuse the view,so I write:
self.layer.contents = nil; then I call the API: [self.layer
setNeedDisplay]; but I got a crash RandomCfllow is my crash:
0 libsystem_kernel.dylib 0x3929908c __psynch_cvwait + 24
1 libsystem_c.dylib 0x391f4f19 pthread_cond_wait + 41
2 QuartzCore 0x32d3b205 CAImageProviderSetCallback + 189
3 QuartzCore 0x32dc600d -[CATiledLayer setContents:] + 73
4 TBClient 0x000933ed -[TBRichTextView drawRichTextContent]
(TBRichTextView.m:114)
now I have no idea about the crash,who can give me a help, thanks!