diff --git a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js
index 9a3a35f6e1f2f..bdce362fde9df 100644
--- a/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js
+++ b/packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js
@@ -134,7 +134,10 @@ const objectType = typeBuilderApi => {
   })
 
   // if all child fields are excluded, this type shouldn't exist.
-  if (!Object.keys(transformedFields).length) {
+  // check null first, otherwise cause:
+  // TypeError: Cannot convert undefined or null to object at Function.keys (<anonymous>)
+  // Also cause wordpress blog site build failure in createSchemaCustomization step
+  if (!transformedFields || !Object.keys(transformedFields).length) {
     return
   }