diff --git a/thin-provisioning/metadata_dumper.cc b/thin-provisioning/metadata_dumper.cc
index b62d131..d80359e 100644
--- a/thin-provisioning/metadata_dumper.cc
+++ b/thin-provisioning/metadata_dumper.cc
@@ -597,6 +597,8 @@ namespace {
 
 					info.nr_mappings = n.get_nr_entries();
 				}
+			} else {
+				fail(info, "not the value size of interest");
 			}
 
 			return info;