diff --git a/epan/proto.c b/epan/proto.c
index ea89ac7fcc..fec13db63d 100644
--- a/epan/proto.c
+++ b/epan/proto.c
@@ -3545,6 +3545,7 @@ proto_tree_add_protocol_format(proto_tree *tree, int hfindex, tvbuff_t *tvb,
 			       gint start, gint length, const char *format, ...)
 {
 	proto_item	  *pi;
+	tvbuff_t	  *protocol_tvb;
 	va_list		   ap;
 	header_field_info *hfinfo;
 	gchar* protocol_rep;
@@ -3555,11 +3556,16 @@ proto_tree_add_protocol_format(proto_tree *tree, int hfindex, tvbuff_t *tvb,
 
 	DISSECTOR_ASSERT_FIELD_TYPE(hfinfo, FT_PROTOCOL);
 
+	/*
+	 * This can throw an exception, so do it before we allocate anything.
+	 */
+	protocol_tvb = (start == 0 ? tvb : tvb_new_subset_length(tvb, start, length));
+
 	pi = proto_tree_add_pi(tree, hfinfo, tvb, start, &length);
 
 	va_start(ap, format);
 	protocol_rep = g_strdup_vprintf(format, ap);
-	proto_tree_set_protocol_tvb(PNODE_FINFO(pi), (start == 0 ? tvb : tvb_new_subset_length(tvb, start, length)), protocol_rep);
+	proto_tree_set_protocol_tvb(PNODE_FINFO(pi), protocol_tvb, protocol_rep);
 	g_free(protocol_rep);
 	va_end(ap);