diff --git a/library/DataDefs.cpp b/library/DataDefs.cpp
index c071ad7d3..00c44d975 100644
--- a/library/DataDefs.cpp
+++ b/library/DataDefs.cpp
@@ -540,7 +540,7 @@ const struct_field_info *DFHack::find_union_tag(const struct_field_info *fields,
     }
 
     auto union_fields = ((struct_identity*)union_field->type)->getFields();
-    if (tag_container_type->getFullName(nullptr) != "vector<bool>" &&
+    if (tag_container_type->getFullName() == "vector<bool>" &&
             union_fields[0].mode != struct_field_info::END &&
             union_fields[1].mode != struct_field_info::END &&
             union_fields[2].mode == struct_field_info::END)