WebClient gives SecurityException

I use MacOSX, Unity3.5 and I do this in Start ()

 WebClient wb = new WebClient();
     
 byte[] mybyteArray = wb.DownloadData("http://translate.google.com/translate_tts?tl=en&q=hello");

I get this error. Why? I have also tried similar lines with wb.DownloadFile with the same (or very similar) result.

 SecurityException: Unexpected error while trying to call method_GetSecurityPolicyBlocking : System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Net.WebException: The remote server returned an error: (404) Not Found.
   at System.Net.HttpWebRequest.CheckFinalStatus (System.Net.WebAsyncResult result) [0x00000] in <filename unknown>:0 
   at System.Net.HttpWebRequest.SetResponseData (System.Net.WebConnectionData data) [0x00000] in <filename unknown>:0 
   --- End of inner exception stack trace ---
   at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
   at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <filename unknown>:0 
   at UnityEngine.UnityCrossDomainHelper+WebRequestPolicyProvider.GetPolicy (System.String policy_url) [0x00000] in <filename unknown>:0 
   at UnityEngine.UnityCrossDomainHelper.GetSecurityPolicy (System.String requesturi_string, IPolicyProvider policyProvider) [0x00000] in <filename unknown>:0 
   at UnityEngine.UnityCrossDomainHelper.GetSecurityPolicyForDotNetWebRequest (System.String requesturi_string, System.Reflection.MethodInfo policyProvidingMethod) [0x00000] in <filename unknown>:0 
   at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (object,object[],System.Exception&)
   at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
   --- End of inner exception stack trace ---
   at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in <filename unknown>:0 
   at System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) [0x00000] in <filename unknown>:0 
   at System.Net.WebConnection.CheckUnityWebSecurity (System.Net.HttpWebRequest request) [0x00000] in <filename unknown>:0 
 System.Net.WebConnection.LoggedThrow (System.Exception e)
 System.Net.WebConnection.CheckUnityWebSecurity (System.Net.HttpWebRequest request)
 System.Net.WebConnection.Connect (System.Net.HttpWebRequest request)
 Rethrow as WebException: Error: ConnectFailure (Unexpected error while trying to call method_GetSecurityPolicyBlocking : System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Net.WebException: The remote server returned an error: (404) Not Found.
 System.Net.HttpWebRequest.CheckFinalStatus (System.Net.WebAsyncResult result)
 System.Net.HttpWebRequest.SetResponseData (System.Net.WebConnectionData data)
 --- End of inner exception stack trace ---
 System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
 System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters)
 UnityEngine.UnityCrossDomainHelper+WebRequestPolicyProvider.GetPolicy (System.String policy_url)
 UnityEngine.UnityCrossDomainHelper.GetSecurityPolicy (System.String requesturi_string, IPolicyProvider policyProvider)
 UnityEngine.UnityCrossDomainHelper.GetSecurityPolicyForDotNetWebRequest (System.String requesturi_string, System.Reflection.MethodInfo policyProvidingMethod)
 System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
 --- End of inner exception stack trace ---
 System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)
 System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters)
 System.Net.WebConnection.CheckUnityWebSecurity (System.Net.HttpWebRequest request) )
 System.Net.HttpWebRequest.EndGetResponse (IAsyncResult asyncResult)
 System.Net.HttpWebRequest.GetResponse ()
 System.Net.WebClient.GetWebResponse (System.Net.WebRequest request)
 System.Net.WebClient.DownloadDataCore (System.Uri address, System.Object userToken)