diff --git a/src/main/java/org/xbib/marc/MarcField.java b/src/main/java/org/xbib/marc/MarcField.java
index e8f4430..4463851 100644
--- a/src/main/java/org/xbib/marc/MarcField.java
+++ b/src/main/java/org/xbib/marc/MarcField.java
@@ -136,6 +136,9 @@ public class MarcField implements Comparable<MarcField> {
     }
 
     public boolean isTagValid() {
+        if (tag == null) {
+            return true;
+        }
         return tag.length() == 3
                 && ((tag.charAt(0) >= '0' && tag.charAt(0) <= '9')
                 || (tag.charAt(0) >= 'A' && tag.charAt(0) <= 'Z'))
@@ -149,6 +152,9 @@ public class MarcField implements Comparable<MarcField> {
         if (isControl()) {
             return true;
         }
+        if (indicator == null) {
+            return true;
+        }
         boolean b = indicator.length() <= 9;
         for (int i = 0; i < indicator.length(); i++) {
             b = indicator.charAt(i) == ' '
@@ -167,6 +173,9 @@ public class MarcField implements Comparable<MarcField> {
         if (isControl()) {
             return true;
         }
+        if (subfieldIds == null) {
+            return true;
+        }
         boolean b = true;
         for (int i = 0; i < subfieldIds.length(); i++) {
             b = subfieldIds.charAt(i) == ' '
diff --git a/src/test/java/org/xbib/marc/ConcurrencyTest.java b/src/test/java/org/xbib/marc/ConcurrencyTest.java
index 8ef7ef3..446e7ff 100644
--- a/src/test/java/org/xbib/marc/ConcurrencyTest.java
+++ b/src/test/java/org/xbib/marc/ConcurrencyTest.java
@@ -20,8 +20,6 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
 
 import org.junit.Test;
-import org.xbib.marc.Marc;
-import org.xbib.marc.MarcXchangeConstants;
 import org.xbib.marc.json.MarcJsonWriter;
 import org.xbib.marc.xml.MarcXchangeWriter;