diff --git a/scripts/lib/CIME/XML/entry_id.py b/scripts/lib/CIME/XML/entry_id.py
index d02e88680bb..2e16e339f38 100644
--- a/scripts/lib/CIME/XML/entry_id.py
+++ b/scripts/lib/CIME/XML/entry_id.py
@@ -43,29 +43,45 @@ def set_default_value(self, vid, val):
 
         return val
 
-    def get_value_match(self, vid, attributes=None, exact_match=False, entry_node=None):
-        # Handle this case:
-        # <entry id ...>
-        #  <values>
-        #   <value A="a1">X</value>
-        #   <value A="a2">Y</value>
-        #   <value A="a3" B="b1">Z</value>
-        #  </values>
-        # </entry>
+    def get_value_match(self, vid, attributes=None, exact_match=False, entry_node=None,
+                        replacement_for_none=None):
+        """Handle this case:
+        <entry id ...>
+         <values>
+          <value A="a1">X</value>
+          <value A="a2">Y</value>
+          <value A="a3" B="b1">Z</value>
+         </values>
+        </entry>
+
+        If replacement_for_none is provided, then: if the found text value would give a
+        None value, instead replace it with the value given by the replacement_for_none
+        argument. (However, still return None if no match is found.) This may or may not
+        be needed, but is in place to maintain some old logic.
+
+        """
 
         if entry_node is not None:
-            value = self._get_value_match(entry_node, attributes, exact_match)
+            value = self._get_value_match(entry_node, attributes, exact_match,
+                                          replacement_for_none=replacement_for_none)
         else:
             node = self.get_optional_child("entry", {"id":vid})
             value = None
             if node is not None:
-                value = self._get_value_match(node, attributes, exact_match)
+                value = self._get_value_match(node, attributes, exact_match,
+                                              replacement_for_none=replacement_for_none)
         logger.debug("(get_value_match) vid {} value {}".format(vid, value))
         return value
 
-    def _get_value_match(self, node, attributes=None, exact_match=False):
+    def _get_value_match(self, node, attributes=None, exact_match=False,
+                         replacement_for_none=None):
         '''
         Note that the component class has a specific version of this function
+
+        If replacement_for_none is provided, then: if the found text value would give a
+        None value, instead replace it with the value given by the replacement_for_none
+        argument. (However, still return None if no match is found.) This may or may not
+        be needed, but is in place to maintain some old logic.
         '''
         # if there is a <values> element - check to see if there is a match attribute
         # if there is NOT a match attribute, then set the default to "first"
@@ -125,7 +141,12 @@ def _get_value_match(self, node, attributes=None, exact_match=False):
                 expect(False,
                        "match attribute can only have a value of 'last' or 'first', value is %s" %match_type)
 
-        return self.text(mnode)
+        text = self.text(mnode)
+        if text is None:
+            # NOTE(wjs, 2021-06-03) I'm not sure when (if ever) this can happen, but I'm
+            # putting this logic here to maintain some old logic, to be safe.
+            text = replacement_for_none
+        return text
 
     def get_node_element_info(self, vid, element_name):
         node = self.get_optional_child("entry", {"id":vid})
diff --git a/scripts/lib/CIME/XML/namelist_definition.py b/scripts/lib/CIME/XML/namelist_definition.py
index 3a50d35dc25..39c2427eb7e 100644
--- a/scripts/lib/CIME/XML/namelist_definition.py
+++ b/scripts/lib/CIME/XML/namelist_definition.py
@@ -156,6 +156,10 @@ def get_group(self, name):
     def add_attributes(self, attributes):
         self._attributes = attributes
 
+    def get_attributes(self):
+        """Return this object's attributes dictionary"""
+        return self._attributes
+
     def get_entry_nodes(self):
         return self._entry_nodes
 
@@ -174,6 +178,9 @@ def set_value(self, vid, value, subgroup=None, ignore_type=True):
         """This function is not implemented."""
         raise TypeError("NamelistDefinition does not support `set_value`.")
 
+    # In contrast to the entry_id version of this method, this version doesn't support the
+    # replacement_for_none argument, because it is hard-coded to ''.
+    # pylint: disable=arguments-differ
     def get_value_match(self, vid, attributes=None, exact_match=True, entry_node=None):
         """Return the default value for the variable named `vid`.
 
@@ -190,11 +197,12 @@ def get_value_match(self, vid, attributes=None, exact_match=True, entry_node=Non
 
         if entry_node is None:
             entry_node = self._nodes[vid]
+        # NOTE(wjs, 2021-06-04) In the following call, replacement_for_none='' may not
+        # actually be needed, but I'm setting it to maintain some old logic, to be safe.
         value = super(NamelistDefinition, self).get_value_match(vid.lower(),attributes=all_attributes, exact_match=exact_match,
-                                                                entry_node=entry_node)
-        if value is None:
-            value = ''
-        else:
+                                                                entry_node=entry_node,
+                                                                replacement_for_none='')
+        if value is not None:
             value =  self._split_defaults_text(value)
 
         return value
@@ -445,4 +453,4 @@ def get_default_value(self, item, attribute=None):
             all_attributes.update(attribute)
 
         value = self.get_value_match(item.lower(), all_attributes, True)
-        return self._split_defaults_text(value)
+        return value
diff --git a/scripts/lib/CIME/namelist.py b/scripts/lib/CIME/namelist.py
index cc76117da85..d6c7647ed66 100644
--- a/scripts/lib/CIME/namelist.py
+++ b/scripts/lib/CIME/namelist.py
@@ -953,7 +953,9 @@ def get_variable_value(self, group_name, variable_name):
         if gn:
             vn = string_in_list(variable_name,self._groups[gn])
             if vn:
-                return self._groups[gn][vn]
+                # Make a copy of the list so that any modifications done by the caller
+                # don't modify the internal values.
+                return self._groups[gn][vn][:]
         return ['']
 
     def get_value(self, variable_name):
diff --git a/scripts/lib/CIME/nmlgen.py b/scripts/lib/CIME/nmlgen.py
index 153a58c260c..c823f92ff57 100644
--- a/scripts/lib/CIME/nmlgen.py
+++ b/scripts/lib/CIME/nmlgen.py
@@ -629,6 +629,8 @@ def add_default(self, name, value=None, ignore_abs_path=None):
         have_value = False
         # Check for existing value.
         current_literals = self._namelist.get_variable_value(group, name)
+        if current_literals != [""]:
+            have_value = True
 
         # Check for input argument.
         if value is not None:
@@ -643,7 +645,8 @@ def add_default(self, name, value=None, ignore_abs_path=None):
             have_value = True
             default_literals = self._to_namelist_literals(name, default)
             current_literals = merge_literal_lists(default_literals, current_literals)
-        expect(have_value, "No default value found for {}.".format(name))
+        expect(have_value, "No default value found for {} with attributes {}.".format(
+            name, self._definition.get_attributes()))
 
         # Go through file names and prepend input data root directory for
         # absolute pathnames.
diff --git a/src/drivers/mct/cime_config/namelist_definition_modelio.xml b/src/drivers/mct/cime_config/namelist_definition_modelio.xml
index 660bc93dee3..ce0275e59bf 100644
--- a/src/drivers/mct/cime_config/namelist_definition_modelio.xml
+++ b/src/drivers/mct/cime_config/namelist_definition_modelio.xml
@@ -172,6 +172,7 @@
     <value component="glc">$GLC_PIO_NETCDF_FORMAT</value>
     <value component="wav">$WAV_PIO_NETCDF_FORMAT</value>
     <value component="iac">$IAC_PIO_NETCDF_FORMAT</value>
+    <value component="esp">$ESP_PIO_NETCDF_FORMAT</value>
     </values>
   </entry>