iOS AOT failing?

The library is working fine in the Editor (purchased through Asset store), but I'm getting this when I build for iOS:

Cross compilation job websocket-sharp.dll failed.
UnityEngine.UnityException: Failed AOT cross compiler: /Applications/Unity/Unity.app/Contents/BuildTargetTools/iPhonePlayer/mono-xcompiler --aot=full,asmonly,nodebug,ficall,static,outfile="websocket-sharp.dll.s" "websocket-sharp.dll" current dir : Temp/StagingArea/Data/Managed
result file exists: True
stdout:
stderr:

at UnityEditor.MonoProcessUtility.RunMonoProcess (System.Diagnostics.Process process, System.String name, System.String resultingFile) [0x00000] in :0
at UnityEditor.MonoCrossCompile.CrossCompileAOT (BuildTarget target, System.String crossCompilerAbsolutePath, System.String assembliesAbsoluteDirectory, CrossCompileOptions crossCompileOptions, System.String input, System.String output, System.String additionalOptions) [0x00000] in :0
at UnityEditor.MonoCrossCompile+JobCompileAOT.ThreadPoolCallback (System.Object threadContext) [0x00000] in :0
UnityEditor.BuildPlayerWindow:BuildPlayerAndRun()

Start with the reported Unity iOS build output and the mono-xcompiler invocation for websocket-sharp.dll in Temp/StagingArea/Data/Managed. Compare the Editor build with the iOS AOT build and determine the cause of the failed cross-compilation. Done means the library builds successfully for iOS without the reported AOT failure.