diff --git a/Sources/ApolloCodegenLib/Templates/CustomScalarTemplate.swift b/Sources/ApolloCodegenLib/Templates/CustomScalarTemplate.swift
index 7d61339cc..c665a47c8 100644
--- a/Sources/ApolloCodegenLib/Templates/CustomScalarTemplate.swift
+++ b/Sources/ApolloCodegenLib/Templates/CustomScalarTemplate.swift
@@ -14,7 +14,7 @@ struct CustomScalarTemplate: TemplateRenderer {
 
   func renderHeaderTemplate(
     nonFatalErrorRecorder: ApolloCodegen.NonFatalError.Recorder
-  ) -> TemplateString {
+  ) -> TemplateString? {
     HeaderCommentTemplate.editableFileHeader(
       fileCanBeEditedTo: "implement advanced custom scalar functionality."
     )