diff --git a/contrib/slime-presentations.el b/contrib/slime-presentations.el
index 5bf5848c8..302b82fa7 100644
--- a/contrib/slime-presentations.el
+++ b/contrib/slime-presentations.el
@@ -217,6 +217,8 @@ Return buffer index and whether a start-tag was found."
                          (buffer (with-current-buffer object 1))
                          (string 0))
                        nil)))
+        (when (= change-point point)
+          (error "Unable to find presentation start"))
         (setq this-presentation (get-text-property change-point
                                                    presentation object))
         (unless this-presentation
@@ -240,6 +242,8 @@ end-tag was found."
                          (buffer (with-current-buffer object (point-max)))
                          (string (length object)))
                        nil)))
+        (when (= change-point point)
+          (error "Unable to find presentation end"))
         (setq point change-point)
         (setq this-presentation (get-text-property point
                                                    presentation object))))