I am getting this error when I am validating the user with sql membership provider

this.provider.ValidateUser(userName, password); the password i have used is "freetrial". I tried trimming the spaces but still no luck!!!

and the call stack is as follows:

[FormatException: Invalid character in a Base-64 string.] System.Convert.FromBase64String(String s) +0 System.Web.Security.MembershipProvider.EncodePassword(String pass, Int32 passwordFormat, String salt) +54 System.Web.Security.SqlMembershipProvider.CheckPassword(String username, String password, Boolean updateLastLoginActivityDate, Boolean failIfNotApproved, String& salt, Int32& passwordFormat) +169 System.Web.Security.SqlMembershipProvider.CheckPassword(String username, String password, Boolean updateLastLoginActivityDate, Boolean failIfNotApproved) +42 System.Web.Security.SqlMembershipProvider.ValidateUser(String username, String password) +78