diff --git a/src/linter/implicit/type_union_implicit.h b/src/linter/implicit/type_union_implicit.h
index 85e98f8..7b5ff0e 100644
--- a/src/linter/implicit/type_union_implicit.h
+++ b/src/linter/implicit/type_union_implicit.h
@@ -24,6 +24,10 @@ class TypeUnionImplicit final : public sourcemeta::alterschema::Rule {
                       "http://json-schema.org/draft-00/hyper-schema#"}) &&
         schema.defines("type")) {
       return false;
+
+      // Don't apply if we don't have the necessary vocabularies
+    } else {
+      return false;
     }
 
     if (vocabularies.contains(