diff --git a/jablib/src/main/java/org/jabref/logic/importer/fetcher/citation/crossref/CrossRefCitationFetcher.java b/jablib/src/main/java/org/jabref/logic/importer/fetcher/citation/crossref/CrossRefCitationFetcher.java
index 57668edf34f..920535bcfb7 100644
--- a/jablib/src/main/java/org/jabref/logic/importer/fetcher/citation/crossref/CrossRefCitationFetcher.java
+++ b/jablib/src/main/java/org/jabref/logic/importer/fetcher/citation/crossref/CrossRefCitationFetcher.java
@@ -87,11 +87,11 @@ public List<BibEntry> getReferences(BibEntry entry) throws FetcherException {
             return references.valueStream()
                              .map(Unchecked.function(reference -> {
                                  String unstructured = reference.at("/unstructured").asText();
-                                 String referenceDoi = reference.at("/DOI").asText();
-                                 if (referenceDoi == null) {
+                                 JsonNode doiNode = reference.at("/DOI");
+                                 if (doiNode.isMissingNode()) {
                                      return getBibEntryFromText(parser, unstructured);
                                  } else {
-                                     return getBibEntryFromDoi(referenceDoi, unstructured);
+                                     return getBibEntryFromDoi(doiNode.asText(), unstructured);
                                  }
                              }))
                              .toList();
diff --git a/jablib/src/main/java/org/jabref/logic/importer/plaincitation/RuleBasedPlainCitationParser.java b/jablib/src/main/java/org/jabref/logic/importer/plaincitation/RuleBasedPlainCitationParser.java
index caba5797f85..99fe72ef85d 100644
--- a/jablib/src/main/java/org/jabref/logic/importer/plaincitation/RuleBasedPlainCitationParser.java
+++ b/jablib/src/main/java/org/jabref/logic/importer/plaincitation/RuleBasedPlainCitationParser.java
@@ -8,17 +8,14 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
-import org.jabref.logic.importer.FetcherException;
 import org.jabref.model.entry.BibEntry;
 import org.jabref.model.entry.field.StandardField;
 import org.jabref.model.entry.types.EntryType;
 import org.jabref.model.entry.types.StandardEntryType;
 
-/**
- * Parse a plain citation using regex rules.
- * <p>
- * TODO: This class is similar to {@link org.jabref.logic.importer.fileformat.pdf.RuleBasedBibliographyPdfImporter}, we need to unify them.
- */
+/// Parse a plain citation using regex rules.
+///
+/// TODO: This class is similar to {@link org.jabref.logic.importer.fileformat.pdf.RuleBasedBibliographyPdfImporter}, we need to unify them.
 public class RuleBasedPlainCitationParser implements PlainCitationParser {
     private static final String AUTHOR_TAG = "[author_tag]";
     private static final String URL_TAG = "[url_tag]";
@@ -52,16 +49,24 @@ public class RuleBasedPlainCitationParser implements PlainCitationParser {
             "(p.)?\\s?\\d+(-\\d+)?",
             Pattern.CASE_INSENSITIVE | Pattern.MULTILINE | Pattern.DOTALL);
 
-    private final List<String> urls = new ArrayList<>();
-    private final List<String> authors = new ArrayList<>();
-    private String year = "";
-    private String pages = "";
-    private String title = "";
-    private boolean isArticle = true;
-    private String journalOrPublisher = "";
+    private List<String> urls;
+    private List<String> authors;
+    private String year;
+    private String pages;
+    private String title;
+    private boolean isArticle;
+    private String journalOrPublisher;
 
     @Override
-    public Optional<BibEntry> parsePlainCitation(String text) throws FetcherException {
+    public Optional<BibEntry> parsePlainCitation(String text) {
+        urls = new ArrayList<>();
+        authors = new ArrayList<>();
+        year = "";
+        pages = "";
+        title = "";
+        isArticle = true;
+        journalOrPublisher = "";
+
         String inputWithoutUrls = findUrls(text);
         String inputWithoutAuthors = findAuthors(inputWithoutUrls);
         String inputWithoutYear = findYear(inputWithoutAuthors);