pkhex won't open

[big man]

Every time I try to open pkhex I'm greeted with an error message saying: System.TypeLoadException: Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
   at System.ModuleHandle.ResolveMethod(RuntimeModule module, Int32 methodToken, IntPtr* typeInstArgs, Int32 typeInstCount, IntPtr* methodInstArgs, Int32 methodInstCount)
   at System.ModuleHandle.ResolveMethodHandleInternalCore(RuntimeModule module, Int32 methodToken, IntPtr[] typeInstantiationContext, Int32 typeInstCount, IntPtr[] methodInstantiationContext, Int32 methodInstCount)
   at System.ModuleHandle.ResolveMethodHandleInternal(RuntimeModule module, Int32 methodToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext)
   at System.Reflection.CustomAttributeData..ctor(RuntimeModule scope, CustomAttributeRecord caRecord)
   at System.Reflection.CustomAttributeData.GetCustomAttributes(RuntimeModule module, Int32 tkTarget)
   at System.Reflection.CustomAttributeData.GetCustomAttributesInternal(RuntimeAssembly target)
   at System.Resources.ManifestBasedResourceGroveler.GetNeutralResourcesLanguage(Assembly a, UltimateResourceFallbackLocation& fallbackLocation)
   at System.Resources.ResourceManager.CommonSatelliteAssemblyInit()
   at System.Resources.ResourceManager..ctor(String baseName, Assembly assembly)
   at PKHeX.WinForms.Properties.Resources.get_ResourceManager()
   at PKHeX.WinForms.SplashScreen.InitializeComponent()
   at PKHeX.WinForms.Program.Main()
System.TypeLoadException: Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
   at System.ModuleHandle.ResolveMethod(RuntimeModule module, Int32 methodToken, IntPtr* typeInstArgs, Int32 typeInstCount, IntPtr* methodInstArgs, Int32 methodInstCount)
   at System.ModuleHandle.ResolveMethodHandleInternalCore(RuntimeModule module, Int32 methodToken, IntPtr[] typeInstantiationContext, Int32 typeInstCount, IntPtr[] methodInstantiationContext, Int32 methodInstCount)
   at System.ModuleHandle.ResolveMethodHandleInternal(RuntimeModule module, Int32 methodToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext)
   at System.Reflection.CustomAttributeData..ctor(RuntimeModule scope, CustomAttributeRecord caRecord)
   at System.Reflection.CustomAttributeData.GetCustomAttributes(RuntimeModule module, Int32 tkTarget)
   at System.Reflection.CustomAttributeData.GetCustomAttributesInternal(RuntimeAssembly target)
   at System.Resources.ManifestBasedResourceGroveler.GetNeutralResourcesLanguage(Assembly a, UltimateResourceFallbackLocation& fallbackLocation)
   at System.Resources.ResourceManager.CommonSatelliteAssemblyInit()
   at System.Resources.ResourceManager..ctor(String baseName, Assembly assembly)
   at PKHeX.WinForms.Properties.Resources.get_ResourceManager()
   at PKHeX.WinForms.Properties.Resources.get_Icon()
   at PKHeX.WinForms.ErrorWindow.InitializeComponent()
   at PKHeX.WinForms.ErrorWindow..ctor(String lang)
   at PKHeX.WinForms.ErrorWindow.ShowErrorDialog(String friendlyMessage, Exception ex, Boolean allowContinue)
   at PKHeX.WinForms.Program.CurrentDomain_UnhandledException(Object sender, UnhandledExceptionEventArgs e)

and I am the least bit tech savvy, so I don't know what this means. Any advise would be helpful.