diff --git a/src/StringTagField.php b/src/StringTagField.php
index 27bf13a..180d7a5 100644
--- a/src/StringTagField.php
+++ b/src/StringTagField.php
@@ -254,12 +254,12 @@ class StringTagField extends DropdownField
     public function setValue($value, $source = null)
     {
         if (is_string($value)) {
-            $value = explode(',', $value);
+            $value = explode(',', $value ?? '');
         }
 
         if ($source instanceof DataObject) {
             $name = $this->getName();
-            $value = explode(',', $source->$name);
+            $value = explode(',', $source->$name ?? '');
         }
 
         if ($source instanceof SS_List) {
@@ -270,7 +270,7 @@ class StringTagField extends DropdownField
             $value = [];
         }
 
-        return parent::setValue(array_filter($value));
+        return parent::setValue(array_filter($value ?? []));
     }
 
     public function saveInto(DataObjectInterface $record)
@@ -328,7 +328,7 @@ class StringTagField extends DropdownField
             /** @var ArrayData $tag */
             $tagValue = $tag->Value;
             // Map into a distinct list (prevent duplicates)
-            if (stripos($tagValue, $term) !== false && !array_key_exists($tagValue, $items)) {
+            if (stripos($tagValue ?? '', $term ?? '') !== false && !array_key_exists($tagValue, $items ?? [])) {
                 $items[$tagValue] = [
                     'id' => $tag->Title,
                     'text' => $tag->Value,
@@ -336,7 +336,7 @@ class StringTagField extends DropdownField
             }
         }
         // @todo do we actually need lazy loading limits for StringTagField?
-        return array_slice(array_values($items), 0, $this->getLazyLoadItemLimit());
+        return array_slice(array_values($items ?? []), 0, $this->getLazyLoadItemLimit());
     }
 
     /**
diff --git a/src/TagField.php b/src/TagField.php
index d3595bc..2c0b2ef 100644
--- a/src/TagField.php
+++ b/src/TagField.php
@@ -234,7 +234,7 @@ class TagField extends MultiSelectField
         );
 
         if (!$this->getShouldLazyLoad()) {
-            $schema['options'] = array_values($this->getOptions()->toNestedArray());
+            $schema['options'] = array_values($this->getOptions()->toNestedArray() ?? []);
         } else {
             $schema['optionUrl'] = $this->getSuggestURL();
         }
@@ -421,7 +421,7 @@ class TagField extends MultiSelectField
             }
         }
 
-        $relation->setByIDList(array_filter($ids));
+        $relation->setByIDList(array_filter($ids ?? []));
     }
 
     /**
@@ -513,7 +513,7 @@ class TagField extends MultiSelectField
             ];
         }
 
-        return array_values($items);
+        return array_values($items ?? []);
     }
 
     /**
diff --git a/tests/TagFieldTest.php b/tests/TagFieldTest.php
index 4e24ea6..9a70742 100755
--- a/tests/TagFieldTest.php
+++ b/tests/TagFieldTest.php
@@ -72,7 +72,7 @@ class TagFieldTest extends SapphireTest
      */
     protected function compareTagLists(array $expected, DataList $actualSource)
     {
-        $actual = array_values($actualSource->map('ID', 'Title')->toArray());
+        $actual = array_values($actualSource->map('ID', 'Title')->toArray() ?? []);
         sort($expected);
         sort($actual);