diff --git a/packages/gatsby/src/schema/types/derived-types.ts b/packages/gatsby/src/schema/types/derived-types.ts
index 3a752d53b8d2c..2c63aee9b1547 100644
--- a/packages/gatsby/src/schema/types/derived-types.ts
+++ b/packages/gatsby/src/schema/types/derived-types.ts
@@ -51,7 +51,7 @@ const getDerivedTypes = ({
   typeComposer,
 }: {
   typeComposer: AllTypeComposer
-}): any => typeComposer.getExtension(`derivedTypes`) || new Set()
+}): Set<string> => typeComposer.getExtension(`derivedTypes`) || new Set()
 
 export const clearDerivedTypes = ({
   schemaComposer,