diff --git a/src/FamilyCustomFieldsEditor.php b/src/FamilyCustomFieldsEditor.php
index 383f5c8c0a..205df418c1 100644
--- a/src/FamilyCustomFieldsEditor.php
+++ b/src/FamilyCustomFieldsEditor.php
@@ -136,7 +136,7 @@
                 // doesn't allow numeric-only field (table column) names.
                 $fields = mysqli_query($cnInfoCentral, 'SELECT * FROM family_custom');
                 $fieldInfo = mysqli_fetch_field_direct($fields, $last);
-                $newFieldNum = mb_substr($fieldInfo->name, 1) + 1;
+                $newFieldNum = (int) mb_substr($fieldInfo->name, 1) + 1;
 
                 // If we're inserting a new custom-list type field,
                 // create a new list and get its ID
diff --git a/src/PersonCustomFieldsEditor.php b/src/PersonCustomFieldsEditor.php
index 79a55c9ac9..d98fd99b38 100644
--- a/src/PersonCustomFieldsEditor.php
+++ b/src/PersonCustomFieldsEditor.php
@@ -127,7 +127,7 @@
                     // The "c#" naming scheme is necessary because MySQL 3.23 doesn't allow numeric-only field (table column) names.
                     $fields = mysqli_query($cnInfoCentral, 'SELECT * FROM person_custom');
                     $fieldInfo = mysqli_fetch_field_direct($fields, $last);
-                    $newFieldNum = mb_substr($fieldInfo->name, 1) + 1;
+                    $newFieldNum = (int) mb_substr($fieldInfo->name, 1) + 1;
 
                     // If we're inserting a new custom-list type field, create a new list and get its ID
                     if ($newFieldType == 12) {