How to convert Image to Memory stream? - xamarin.ios

Question

I am new to xamarin. I am designing an application in which we are using SignaturePad. The signature is converting to byte using memory stream and is working fine in Android.But in IOS i am getting exception Specified cast not valid or Invalid cast Exception.Below is the code i implemented.
SignaturepadView.Xaml code:
<Grid VerticalOptions="Center">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<signature:SignaturePadView x:Name="padView"
Grid.Row="0"
Grid.ColumnSpan="2"
HeightRequest="100"
WidthRequest="240"
BackgroundColor="#887e70"
PromptText=""
CaptionText=""
PromptTextColor="Red"
StrokeColor="#ece6dd"
StrokeWidth="3" />
<Button Grid.Row="1"
Grid.Column="0"
Text="Save Signature"
Clicked="OnSaveClicked"/>
<Button Grid.Row="1"
Grid.Column="1"
Text="Clear"
Clicked="OnClearClicked"/>
</Grid>
In Xaml.cs
private async void OnSaveClicked(object sender, EventArgs e)
{
var format = SignatureImageFormat.Png;
var image = await padView.GetImageStreamAsync(format);`
var signatureMemoryStream = (MemoryStream)image;
byte[] data = signatureMemoryStream.ToArray();
DependencyService.Get<IPicture>().SavePictureToDisk("signimage", data);
}
This is working fine in Android.But i am getting exception Specified cast not valid or Invalid cast Exception for IOS. Am i doing anything wrong?. Please help me. Thanks in Advance.
Note: I installed package https://components.xamarin.com/view/signature-pad in PCL,Android and IOS as well. Sorry for my English.