diff --git a/utils/string-utils.class.ts b/utils/string-utils.class.ts
index 8c6577c4..d05bc57e 100644
--- a/utils/string-utils.class.ts
+++ b/utils/string-utils.class.ts
@@ -220,8 +220,8 @@ export class Identifier {
           }
         }
       }
-      return null;
     }
+    return null;
   }
   
   public static isValidDOI(str: string): boolean {