Ue4 Crashes When i Import a FBX

When i try to import my FBX the engine crashes and it gives me a error. Why is it doing this.

MachineId:A8536B6448F9906FD16AE7B6C2533B44
EpicAccountId:6bc7170d05a445df934c978c439cfa8e

Unknown exception - code 00000001 (first/second chance not available)

Assertion failed: ChartB.Join[ Side ^ 1 ] == i [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.6\Engine\Source\Developer\MeshUtilities\Private\LayoutUV.cpp] [Line: 374] 

KERNELBASE + 37901 bytes
UE4Editor_Core + 3092349 bytes
UE4Editor_Core + 1597688 bytes
UE4Editor_Core + 1479058 bytes
UE4Editor_MeshUtilities + 222402 bytes
UE4Editor_MeshUtilities + 149371 bytes
UE4Editor_Engine + 8442069 bytes
UE4Editor_Engine + 8443260 bytes
UE4Editor_Engine + 8423259 bytes
UE4Editor_UnrealEd + 9575680 bytes
UE4Editor_UnrealEd + 9136826 bytes
UE4Editor_UnrealEd + 9270727 bytes
UE4Editor_AssetTools + 847862 bytes
UE4Editor_AssetTools + 841823 bytes
UE4Editor_ContentBrowser + 1614625 bytes
UE4Editor_ContentBrowser + 87303 bytes
UE4Editor_ContentBrowser + 121209 bytes
UE4Editor_ContentBrowser + 1344569 bytes
UE4Editor_Slate + 2116506 bytes
UE4Editor_Slate + 2296435 bytes
UE4Editor_Slate + 432060 bytes
UE4Editor_Slate + 160583 bytes
UE4Editor_Slate + 832224 bytes
UE4Editor_Slate + 770598 bytes
UE4Editor_Core + 2484713 bytes
UE4Editor_Core + 2400763 bytes
UE4Editor_Core + 2489275 bytes
UE4Editor_Core + 2387186 bytes
user32 + 105425 bytes
user32 + 104666 bytes
UE4Editor_Core + 2490486 bytes
UE4Editor!FEngineLoop::Tick() + 3434 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.6\engine\source\runtime\launch\private\launchengineloop.cpp:2193]
UE4Editor!GuardedMain() + 479 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.6\engine\source\runtime\launch\private\launch.cpp:131]
UE4Editor!GuardedMainWrapper() + 26 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.6\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
UE4Editor!WinMain() + 249 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.6\engine\source\runtime\launch\private\windows\launchwindows.cpp:202]
UE4Editor!__tmainCRTStartup() + 329 bytes [f:\dd\vctools\crt\crtw32\dllstuff\crtexe.c:618]

Does anyone know what causing this?.