The glue code expects a zeroed-out gvalue, so to provide this we must initialize it in managed code and pass it as a ref rather than out.

--- gsf-sharp-0.5/gsf/DocProp.custom	2005-10-28 17:02:40.000000000 +0100
+++ gsf-sharp-dsd/gsf/DocProp.custom	2005-11-02 13:54:59.000000000 +0000
@@ -1,6 +1,6 @@
 
 		[DllImport("libgsfglue")]
-		static extern void gsf_doc_prop_glue_get_val(IntPtr raw, out GLib.GType type, out GLib.Value gvalue);
+		static extern void gsf_doc_prop_glue_get_val(IntPtr raw, out GLib.GType type, ref GLib.Value gvalue);
 
 		[DllImport("libgsfglue")]
 		static extern void gsf_doc_prop_glue_set_val(IntPtr raw, ref GLib.Value val);
@@ -8,9 +8,9 @@
 		public object Val { 
 			get {
 				GLib.GType type = GLib.GType.None;
-				GLib.Value gvalue;
+				GLib.Value gvalue = GLib.Value.Empty;
 
-				gsf_doc_prop_glue_get_val(Handle, out type, out gvalue);
+				gsf_doc_prop_glue_get_val(Handle, out type, ref gvalue);
 
 				if (type != GLib.GType.None)
 					return gvalue.Val;