diff --git a/java/src/org/broadinstitute/sting/utils/cmdLine/ArgumentTypeDescriptor.java b/java/src/org/broadinstitute/sting/utils/cmdLine/ArgumentTypeDescriptor.java
index 9c7db1d01..d2ee0e057 100644
--- a/java/src/org/broadinstitute/sting/utils/cmdLine/ArgumentTypeDescriptor.java
+++ b/java/src/org/broadinstitute/sting/utils/cmdLine/ArgumentTypeDescriptor.java
@@ -278,7 +278,12 @@ class SimpleArgumentTypeDescriptor extends ArgumentTypeDescriptor {
                     catch (NoSuchFieldException e) { throw new StingException("parsing " + type.toString() + "doesn't contain the field " + val.toString()); }
                 }
                 // if their argument has no value (null), and there's a default, return that default for the enum value
-                if (defaultEnumeration != null && value == null) return defaultEnumeration;
+                if (defaultEnumeration != null && value == null)
+                    return defaultEnumeration;
+                // if their argument has no value and there's no default, throw a missing argument value exception.
+                // TODO: Clean this up so that null values never make it to this point.  To fix this, we'll have to clean up the implementation of -U.
+                if (value == null)
+                    throw new MissingArgumentValueException(Collections.singleton(createDefaultArgumentDefinition(source)));
                 throw new UnknownEnumeratedValueException(value, type.getName());
             } else {
                 Constructor ctor = type.getConstructor(String.class);
diff --git a/java/src/org/broadinstitute/sting/utils/cmdLine/CommandLineProgram.java b/java/src/org/broadinstitute/sting/utils/cmdLine/CommandLineProgram.java
index 20ff5b309..eb56e63aa 100644
--- a/java/src/org/broadinstitute/sting/utils/cmdLine/CommandLineProgram.java
+++ b/java/src/org/broadinstitute/sting/utils/cmdLine/CommandLineProgram.java
@@ -27,7 +27,7 @@ import java.util.Enumeration;
  * <p/>
  * <p/>
  * This class is our implementation of the command line parser, similar to Pickard's.  We instead
- * support GNU style command line arguements, and use this class to setup the global parser.
+ * support GNU style command line arguments, and use this class to setup the global parser.
  */
 public abstract class CommandLineProgram {