nan values during rendering (HairBxDF?)

[mmp]

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I'm using a realistic camera by importing some lens parameters. I'm rendering a typical pbrt scene with many trees and grass. However very often I run into the following error. I'm not quite sure what this means and how to potentially fix it. Has anyone encountered this before? Any help would be greatly appreciated!

Rendering: [+++++++++ ] (211.6s|790.2s) F0822 23:06:23.561084 1083064 path.cpp:140] Check failed: beta.y() >= 0.f (-nan vs. 0)
*** Check failure stack trace: ***
@ 0x55f3cf063b19 pbrt::ScrambledRadicalInverse()
@ 0x55f3cf060e30 pbrt::RadicalInverse()
@ 0x55f3cf0642a9 pbrt::ScrambledRadicalInverse()
@ 0x55f3cee519fb pbrt::ReadImage()
@ 0x55f3cef1924c pbrt::HairBSDF::Sample_f()
@ 0x55f3cedbda24 (unknown)
@ 0x55f3cedbe102 (unknown)
@ 0x7efd06cd44a3 (unknown)
@ 0x7efd069c741c (unknown)
@ 0x7efd06a477ac (unknown)
@ (nil) (unknown)