Error when enumerating through claims collection by MonkeyTennis

Intermittently we see the following error when browsing a website:

Collection was modified; enumeration operation may not execute.
Exception type:
System.InvalidOperationException
StackTrace:
at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
at Microsoft.IdentityModel.Claims.ClaimCollection.CopyWithSubject(IClaimsIdentity subject)
at Microsoft.IdentityModel.Claims.WindowsClaimsIdentity.Copy()
at Microsoft.IdentityModel.Claims.WindowsClaimsPrincipal..ctor(WindowsClaimsIdentity identity)
at Microsoft.IdentityModel.Claims.ClaimsPrincipal.CreateFromIdentity(IIdentity identity, String windowsIssuerName)
at Microsoft.IdentityModel.Claims.ClaimsPrincipal.CreateFromIdentities(ClaimsIdentityCollection identities, String windowsIssuerName)
at Microsoft.IdentityModel.Web.SessionAuthenticationModule.SetPrincipalFromSessionToken(SessionSecurityToken sessionSecurityToken)
at Microsoft.IdentityModel.Web.SessionAuthenticationModule.AuthenticateSessionSecurityToken(SessionSecurityToken sessionToken, Boolean writeCookie)
at Microsoft.IdentityModel.Web.SessionAuthenticationModule.OnAuthenticateRequest(Object sender, EventArgs eventArgs)
at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Note this error is occurring within Windows Identity Foundation itself, not our custom code. We do have a SessionSecurityTokenReceived event handler which modifies the claims collection and so I suspect that this is happening at the same time as the stack trace above is executing. However, I can't think of a scenario where both might happen at the same time. regardless, WIF should not error in this way.

Would the fix be to use this._claims.ToList() rather than this._claims, in Microsoft.IdentityModel.Claims.ClaimCollection.CopyWithSubject().