diff --git a/collects/scribble/scheme.ss b/collects/scribble/scheme.ss
index 98b2a200..dba8b5f8 100644
--- a/collects/scribble/scheme.ss
+++ b/collects/scribble/scheme.ss
@@ -312,6 +312,12 @@
             (advance c init-line!)]
            [(and (pair? (syntax-e c))
                  (eq? (syntax-e (car (syntax-e c))) 'code:comment))
+            (let ([l (syntax->list c)])
+              (unless (and l (= 2 (length l)))
+                (raise-syntax-error
+                 #f
+                 "does not have a single sub-form"
+                 c)))
             (advance c init-line!)
             (out "; " comment-color)
             (let ([v (syntax->datum (cadr (syntax->list c)))])
diff --git a/collects/scribblings/scribble/manual.scrbl b/collects/scribblings/scribble/manual.scrbl
index 9fe94818..43351309 100644
--- a/collects/scribblings/scribble/manual.scrbl
+++ b/collects/scribblings/scribble/manual.scrbl
@@ -122,7 +122,10 @@ without insetting the code.}
 
 @defform[(schememod lang datum ...)]{Like @scheme[schemeblock], but
 the @scheme[datum] are typeset inside a @schememodfont{#lang}-form
-module whose language is @scheme[lang].}
+module whose language is @scheme[lang]. The source location of
+@scheme[lang] (relative to the body @scheme[datum]s) determines the
+relative positioning of the @schememodfont{#lang} line in the typeset
+output.}
 
 @defform[(scheme datum ...)]{Like @scheme[schemeblock], but typeset on
 a single line and wrapped with its enclosing paragraph, independent of