diff --git a/collects/net/cgi-unit.ss b/collects/net/cgi-unit.ss
index b9211402c2..e95926ed46 100644
--- a/collects/net/cgi-unit.ss
+++ b/collects/net/cgi-unit.ss
@@ -206,7 +206,10 @@
   ;; get-cgi-method : () -> string
   ;; -- string is either GET or POST (though future extension is possible)
   (define (get-cgi-method)
-    (getenv "REQUEST_METHOD"))
+    (let ([request-method (getenv "REQUEST_METHOD")])
+      (if (string? request-method)
+          request-method
+          (error 'get-cgi-method "REQUEST_METHOD environment variable: expected string, got ~v" request-method))))
 
   ;; generate-link-text : string x html-string -> html-string
   (define (generate-link-text url anchor-text)