diff --git a/autoload/nerdtree.vim b/autoload/nerdtree.vim
index be1afbe..f3fc0be 100644
--- a/autoload/nerdtree.vim
+++ b/autoload/nerdtree.vim
@@ -101,9 +101,7 @@ endfunction
 "FUNCTION: nerdtree#closeTree() {{{2
 "Closes the primary NERD tree window for this tab
 function! nerdtree#closeTree()
-    if !nerdtree#isTreeOpen()
-        throw "NERDTree.NoTreeFoundError: no NERDTree is open"
-    endif
+    call g:NERDTree.MustBeOpen()
 
     if winnr("$") != 1
         if winnr() == g:NERDTree.GetWinNum()
@@ -125,7 +123,7 @@ endfunction
 "FUNCTION: nerdtree#closeTreeIfOpen() {{{2
 "Closes the NERD tree window if it is open
 function! nerdtree#closeTreeIfOpen()
-   if nerdtree#isTreeOpen()
+   if g:NERDTree.IsOpen()
       call nerdtree#closeTree()
    endif
 endfunction
@@ -133,7 +131,7 @@ endfunction
 "FUNCTION: nerdtree#closeTreeIfQuitOnOpen() {{{2
 "Closes the NERD tree window if the close on open option is set
 function! nerdtree#closeTreeIfQuitOnOpen()
-    if g:NERDTreeQuitOnOpen && nerdtree#isTreeOpen()
+    if g:NERDTreeQuitOnOpen && g:NERDTree.IsOpen()
         call nerdtree#closeTree()
     endif
 endfunction
@@ -275,11 +273,6 @@ function! nerdtree#echoWarning(msg)
     echohl normal
 endfunction
 
-"FUNCTION: nerdtree#isTreeOpen() {{{2
-function! nerdtree#isTreeOpen()
-    return g:NERDTree.GetWinNum() != -1
-endfunction
-
 "FUNCTION: nerdtree#putCursorOnBookmarkTable(){{{2
 "Places the cursor at the top of the bookmarks table
 function! nerdtree#putCursorOnBookmarkTable()
@@ -306,10 +299,7 @@ endfunction
 "FUNCTION: nerdtree#putCursorInTreeWin(){{{2
 "Places the cursor in the nerd tree window
 function! nerdtree#putCursorInTreeWin()
-    if !nerdtree#isTreeOpen()
-        throw "NERDTree.InvalidOperationError: cant put cursor in NERD tree window, no window exists"
-    endif
-
+    call g:NERDTree.MustBeOpen()
     call nerdtree#exec(g:NERDTree.GetWinNum() . "wincmd w")
 endfunction
 
diff --git a/autoload/nerdtree/ui_glue.vim b/autoload/nerdtree/ui_glue.vim
index a86011c..3b9784a 100644
--- a/autoload/nerdtree/ui_glue.vim
+++ b/autoload/nerdtree/ui_glue.vim
@@ -275,7 +275,7 @@ function! s:findAndRevealPath()
         endif
     else
         if !p.isUnder(g:NERDTreeFileNode.GetRootForTab().path)
-            if !nerdtree#isTreeOpen()
+            if !g:NERDTree.IsOpen()
                 call g:NERDTreeCreator.TogglePrimary('')
             else
                 call nerdtree#putCursorInTreeWin()
@@ -283,7 +283,7 @@ function! s:findAndRevealPath()
             let b:NERDTreeShowHidden = g:NERDTreeShowHidden
             call s:chRoot(g:NERDTreeDirNode.New(p.getParent()))
         else
-            if !nerdtree#isTreeOpen()
+            if !g:NERDTree.IsOpen()
                 call g:NERDTreeCreator.TogglePrimary("")
             endif
         endif
diff --git a/lib/nerdtree/creator.vim b/lib/nerdtree/creator.vim
index 0697759..efa9846 100644
--- a/lib/nerdtree/creator.vim
+++ b/lib/nerdtree/creator.vim
@@ -55,7 +55,7 @@ function! s:Creator.createPrimary(name)
     endif
 
     if g:NERDTree.ExistsForTab()
-        if nerdtree#isTreeOpen()
+        if g:NERDTree.IsOpen()
             call nerdtree#closeTree()
         endif
         unlet t:NERDTreeBufName
@@ -168,7 +168,7 @@ function! s:Creator.createMirror()
         return
     endif
 
-    if g:NERDTree.ExistsForTab() && nerdtree#isTreeOpen()
+    if g:NERDTree.ExistsForTab() && g:NERDTree.IsOpen()
         call nerdtree#closeTree()
     endif
 
@@ -333,7 +333,7 @@ endfunction
 "initialized.
 function! s:Creator.togglePrimary(dir)
     if g:NERDTree.ExistsForTab()
-        if !nerdtree#isTreeOpen()
+        if !g:NERDTree.IsOpen()
             call self._createTreeWin()
             if !&hidden
                 call b:NERDTree.render()
diff --git a/lib/nerdtree/nerdtree.vim b/lib/nerdtree/nerdtree.vim
index 91ccc53..29a5930 100644
--- a/lib/nerdtree/nerdtree.vim
+++ b/lib/nerdtree/nerdtree.vim
@@ -33,6 +33,19 @@ function! s:NERDTree.GetWinNum()
     endif
 endfunction
 
+"FUNCTION: s:NERDTree.IsOpen() {{{1
+function! s:NERDTree.IsOpen()
+    return s:NERDTree.GetWinNum() != -1
+endfunction
+
+"FUNCTION: s:NERDTree.MustBeOpen() {{{1
+function! s:NERDTree.MustBeOpen()
+    if !s:NERDTree.IsOpen()
+        throw "NERDTree.TreeNotOpen"
+    endif
+endfunction
+
+"FUNCTION: s:NERDTree.New() {{{1
 function! s:NERDTree.New(path)
     let newObj = copy(self)
     let newObj.ui = g:NERDTreeUI.New(newObj)
diff --git a/plugin/NERD_tree.vim b/plugin/NERD_tree.vim
index 28fd3ad..8038668 100644
--- a/plugin/NERD_tree.vim
+++ b/plugin/NERD_tree.vim
@@ -185,7 +185,7 @@ function! NERDTreeRender()
 endfunction
 
 function! NERDTreeFocus()
-    if nerdtree#isTreeOpen()
+    if g:NERDTree.IsOpen()
         call nerdtree#putCursorInTreeWin()
     else
         call g:NERDTreeCreator.TogglePrimary("")