Index: src/main/java/org/apache/logging/log4j/core/pattern/NdcPatternConverter.java
===================================================================
--- src/main/java/org/apache/logging/log4j/core/pattern/NdcPatternConverter.java	(revision 1617640)
+++ src/main/java/org/apache/logging/log4j/core/pattern/NdcPatternConverter.java	(working copy)
@@ -52,7 +52,19 @@
    * {@inheritDoc}
    */
   @Override
-public void format(final LogEvent event, final StringBuilder toAppendTo) {
-    toAppendTo.append(event.getContextStack());
+  public void format(final LogEvent event, final StringBuilder toAppendTo) {
+    // Concatenate stack items with space
+    final StringBuilder sb = new StringBuilder(100);
+    for (final String item : event.getContextStack()) {
+      if (item == null || item.isEmpty()) {
+        continue;
+      }
+      sb.append(item).append(' ');
+    }
+    if(sb.length() > 0)
+    {
+      sb.setLength(sb.length() - 1);
+    }
+    toAppendTo.append(sb.toString());
   }
 }