diff --git a/scheme/httpd/surflets/surflet-input-fields.scm b/scheme/httpd/surflets/surflet-input-fields.scm
index 0b0332d..23c5078 100644
--- a/scheme/httpd/surflets/surflet-input-fields.scm
+++ b/scheme/httpd/surflets/surflet-input-fields.scm
@@ -512,8 +512,12 @@
 (define (image-button-transformer image-button bindings)
   (let ((x (find-image-button-coordinate image-button bindings ".x"))
 	(y (find-image-button-coordinate image-button bindings ".y")))
-    (and x y (cons (string->number x)
-		   (string->number y)))))
+    (let ((x-number (string->number x))
+	  (y-number (string->number y))) 
+    (and x y
+	 (if (and x-number y-number)
+	     (cons x-number y-number)
+	     (error "Image button coordinates aren't numbers. " x y))))))
 
 (define (find-image-button-coordinate image-button bindings suffix)
   (let* ((name (input-field-name image-button)))