diff --git a/MagickCore/channel.h b/MagickCore/channel.h
index 405759d..cd6d313 100644
--- a/MagickCore/channel.h
+++ b/MagickCore/channel.h
@@ -30,7 +30,7 @@
 
 extern MagickExport MagickBooleanType
   GetImageAlphaChannel(const Image *),
-  SetImageAlphaChannel(Image *,const AlphaChannelType,ExceptionInfo *);
+  SetImageAlphaChannel(Image *,const AlphaChannelOption,ExceptionInfo *);
 
 #if defined(__cplusplus) || defined(c_plusplus)
 }