diff --git a/pkgs/at-exp-lib/at-exp/lang/reader.rkt b/pkgs/at-exp-lib/at-exp/lang/reader.rkt
index 7fbb71698e..14285738e7 100644
--- a/pkgs/at-exp-lib/at-exp/lang/reader.rkt
+++ b/pkgs/at-exp-lib/at-exp/lang/reader.rkt
@@ -44,6 +44,7 @@
             (try-dynamic-require 'syntax-color/scribble-lexer 'scribble-lexer)]
            [(definitions-text-surrogate)
             'scribble/private/indentation]
+           [(drracket:default-extension) "scrbl"]
            [(drracket:indentation)
             (dynamic-require 'scribble/private/indentation 'determine-spaces)]
            [else (fallback)]))))))
diff --git a/pkgs/at-exp-lib/scribble/base/reader.rkt b/pkgs/at-exp-lib/scribble/base/reader.rkt
index 2e3ad28608..3f5f848eac 100644
--- a/pkgs/at-exp-lib/scribble/base/reader.rkt
+++ b/pkgs/at-exp-lib/scribble/base/reader.rkt
@@ -30,6 +30,7 @@
        'scribble/private/indentation]
       [(drracket:indentation)
        (dynamic-require 'scribble/private/indentation 'determine-spaces)]
+      [(drracket:default-extension) "scrbl"]
       [else (default key defval)])))
 
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