diff --git a/src/mu-msg-part.c b/src/mu-msg-part.c
index d2ec1648..bfbb5c16 100644
--- a/src/mu-msg-part.c
+++ b/src/mu-msg-part.c
@@ -130,7 +130,8 @@ mu_msg_part_get_text (MuMsgPart *self, gboolean *err)
 }
 
 
-
+/* note: this will return -1 in case of error or if the size is
+ * unknown */
 static ssize_t
 get_part_size (GMimePart *part)
 {
@@ -143,11 +144,11 @@ get_part_size (GMimePart *part)
 
 	stream = g_mime_data_wrapper_get_stream (wrapper);
 	if (!stream)
-		return -1;
+		return -1; /* no stream -> size is 0 */
+	else
+		return g_mime_stream_length (stream);
 
 	/* NOTE: it seems we shouldn't unref stream/wrapper */
-
-	return g_mime_stream_length (stream);
 }
 
 
diff --git a/src/mu-msg-part.h b/src/mu-msg-part.h
index c0500e11..fa2c9d4d 100644
--- a/src/mu-msg-part.h
+++ b/src/mu-msg-part.h
@@ -49,8 +49,8 @@ struct _MuMsgPart {
 	/* usually, "attachment" or "inline" */
 	char             *disposition;
 
-	/* size of the part; or <= 0 if unknown */
-	size_t		 size;
+	/* size of the part; or < 0 if unknown */
+	ssize_t		 size;
 
 	gpointer         data; /* opaque data */
 
diff --git a/src/mu-msg-sexp.c b/src/mu-msg-sexp.c
index d1a581c4..804fe0b3 100644
--- a/src/mu-msg-sexp.c
+++ b/src/mu-msg-sexp.c
@@ -222,11 +222,11 @@ each_part (MuMsg *msg, MuMsgPart *part, gchar **parts)
 		name = g_strdup_printf ("\"part-%d\"", part->index);
 
 	tmp = g_strdup_printf
-		("%s(:index %d :name %s :mime-type \"%s/%s\" :size %u)",
+		("%s(:index %d :name %s :mime-type \"%s/%s\" :size %i)",
 		 *parts ? *parts : "",  part->index, name,
 		 part->type ? part->type : "application",
 		 part->subtype ? part->subtype : "octet-stream",
-		 (unsigned)part->size);
+		 (int)part->size);
 
 	g_free (*parts);
 	*parts = tmp;