title: error packing field 61 (org.jpos.iso.ISOException: java.lang.NullPointerException (java.lang.NullPointerException))
body: <p>I am using the mastercard interface specification to pack field 61 like so:</p>
  
  <pre><code>&lt;isofieldpackager
      id="61"
      length="40"
      name="Point-of-Service (POS) Data"
      class="org.jpos.iso.IFB_LLBINARY"
      emitBitmap="false"
      packager="org.jpos.iso.packager.GenericSubFieldPackager"&gt;
      &lt;isofield
          id="1"
          length="1"
          name="POS Terminal Attendance"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="3"
          length="1"
          name="POS Terminal Location"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="4"
          length="1"
          name="POS Cardholder Presence"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="5"
          length="1"
          name="POS Card Presence"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="6"
          length="1"
          name="POS Card Capture Capabilities"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="7"
          length="1"
          name="POS Transaction Status"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="8"
          length="1"
          name="POS Transaction Security"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="10"
          length="1"
          name="Cardholder-Activated Terminal Level"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="11"
          length="1"
          name="POS Card Data Terminal Input Capability Indicator"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="12"
          length="2"
          name="POS Card Data Terminal Input Capability Indicator"
          class="org.jpos.iso.IF_CHAR"/&gt;
      &lt;isofield
          id="13"
          length="3"
          name="POS Country Code"
          class="org.jpos.iso.IF_CHAR"/&gt;
  &lt;/isofieldpackager&gt;
  </code></pre>
  
  <p>And then I am calling the packager like so:</p>
  
  <pre><code>ISOMsg message = new ISOMsg();
  message.set("61.1", "1");       //Unattended terminal
      message.set("61.3", "2");       //Off premises of card acceptor facility
      message.set("61.4", "5");       //Electronic order (home PC, Internet, mobile phone, PDA)
      message.set("61.5", "1");       //Card not present
      message.set("61.6", "0");       //Terminal/operator has no card capture capability
      message.set("61.7", "0");       //Normal request (original presentment)
      message.set("61.8", "0");       //No security concern
      message.set("61.10", "6");      //Authorized Level 6 CAT: Electronic commerce
      message.set("61.11", "6");      //Key entry only
      message.set("61.12", "00");
      message.set("61.13", "716");    //country code Zimbabwe
  </code></pre>
  
  <p>But then I am getting an excetion which looks like this:</p>
  
  <pre><code>    &lt;log realm="channel/127.0.0.1:1234" at="2018-02-20T13:02:14.775" lifespan="31ms"&gt;
    &lt;connect&gt;
  Exception in thread "main" org.jpos.iso.ISOException: error packing field 61 (org.jpos.iso.ISOException: java.lang.NullPointerException (java.lang.NullPointerException))
      at org.jpos.iso.ISOBasePackager.pack(ISOBasePackager.java:184)
      at org.jpos.iso.ISOMsg.pack(ISOMsg.java:456)
      at com.jpos.test.Test.main(Test.java:51)
  Nested:org.jpos.iso.ISOException: java.lang.NullPointerException (java.lang.NullPointerException)
      at org.jpos.iso.packager.GenericSubFieldPackager.pack(GenericSubFieldPackager.java:167)
      at org.jpos.iso.ISOMsgFieldPackager.pack(ISOMsgFieldPackager.java:60)
      at org.jpos.iso.ISOBasePackager.pack(ISOBasePackager.java:175)
      at org.jpos.iso.ISOMsg.pack(ISOMsg.java:456)
      at com.jpos.test.Test.main(Test.java:51)
  Nested:java.lang.NullPointerException
      at org.jpos.iso.packager.GenericSubFieldPackager.pack(GenericSubFieldPackager.java:136)
      at org.jpos.iso.ISOMsgFieldPackager.pack(ISOMsgFieldPackager.java:60)
      at org.jpos.iso.ISOBasePackager.pack(ISOBasePackager.java:175)
      at org.jpos.iso.ISOMsg.pack(ISOMsg.java:456)
      at com.jpos.test.Test.main(Test.java:51)
  /home/terrence/.cache/netbeans/8.2/executor-snippets/run.xml:53: Java returned: 1
  BUILD FAILED (total time: 1 second)
  </code></pre>
  
  <p>Can you please help me understand why I am getting this error, I have tried changing the data classes on the packager, and changing the generic sub-field packager to use the europay subfield packager and the respective data classes but i still get the same exeption</p>