diff --git a/knots/web-server.scm b/knots/web-server.scm
index 58c1883..0c2d3fb 100644
--- a/knots/web-server.scm
+++ b/knots/web-server.scm
@@ -206,8 +206,25 @@ on the procedure being called at any particular time."
                                                stack
                                                (current-error-port)
                                                2)))))))
-                   (lambda (response body)
-                     (sanitize-response request response body))))
+                   (match-lambda*
+                     ((response body)
+                      (sanitize-response request response body))
+                     (other
+                      (let ((stack (make-stack #t))
+                            (exception
+                             (make-exception-with-irritants
+                              (list (make-exception-with-message
+                                     (simple-format
+                                      #f
+                                      "wrong number of values returned from handler, expecting 2, got ~A"
+                                      (length other)))
+                                    handler))))
+                        (print-exception
+                         (current-error-port)
+                         (stack-ref stack 2)
+                         '%exception
+                         (list exception))
+                        (raise-exception exception))))))
                #:unwind? #t)))))
       (with-exception-handler
           write-response-exception-handler