diff --git a/mu4e/mu4e-headers.el b/mu4e/mu4e-headers.el
index 382acbc8..1d6c50e0 100644
--- a/mu4e/mu4e-headers.el
+++ b/mu4e/mu4e-headers.el
@@ -1360,7 +1360,7 @@ maildir)."
     (mu4e-mark-handle-when-leaving)
     (mu4e-headers-search (concat "\"maildir:" maildir "\""))))
 
-(defun mu4e-headers-split-view-grow (n)
+(defun mu4e-headers-split-view-grow (&optional n)
   "In split-view, grow the headers window.
 In horizontal split-view, increase the number of lines shown by N.
 In vertical split-view, increase the number of columns shown by N.
@@ -1380,14 +1380,14 @@ do nothing."
 	   (window-resize hwin n t)
 	   (incf mu4e-headers-visible-columns n)))))))
 
-(defun mu4e-headers-split-view-shrink (n)
+(defun mu4e-headers-split-view-shrink (&optional n)
   "In split-view, shrink the headers window.
 In horizontal split-view, decrease the number of lines shown by N.
 In vertical split-view, decrease the number of columns shown by N.
-If N is negative grow the headers window.
-When not in split-view do nothing."
+If N is negative grow the headers window.  When not in split-view
+do nothing."
   (interactive "P")
-  (mu4e-headers-split-view-grow (- n)))
+  (mu4e-headers-split-view-grow (- (or n 1))))
 
 (defun mu4e-headers-action ()
   "Ask user what to do with message-at-point, then do it.