System.TypeLoadException:EncryptCallDll has no implementation (no RVA). exception while using c++ wrapper class in asp web service

I am using a dll in my web service(asp) like this:

 [DllImport("CEracom.dll", EntryPoint = "InitializeCall")]
      public static extern int InitializeCallDll(string szSlotName, string szPassword);
      [DllImport(@"CEracom.dll", EntryPoint = "EncryptCall")]
      public static extern int EncryptCallDll(byte[] inputString, int len, byte[] arr, string keyName);
      [WebMethod]
      public string calculateKey(string keyIMK, string pan, string panSeqNo)
      {

}

getting exception System.TypeLoadException:

EncryptCallDll

has no implementation (no RVA).Any suggestions?Kept dll in bin folders.

Thanks,

Faiz