diff --git a/src/vs/base/parts/tree/browser/treeViewModel.ts b/src/vs/base/parts/tree/browser/treeViewModel.ts
index e83d595985c66fb80eaa20d92cd8b7a476a9c6a2..e950653ef633852a9872ee154aec6e9541b91dc3 100644
--- a/src/vs/base/parts/tree/browser/treeViewModel.ts
+++ b/src/vs/base/parts/tree/browser/treeViewModel.ts
@@ -45,6 +45,7 @@ export class HeightMap extends EventEmitter {
 			viewItem = this.heightMap[i - 1];
 
 			if (!viewItem) {
+				console.error('view item doesnt exist');
 				return;
 			}
 
@@ -101,6 +102,7 @@ export class HeightMap extends EventEmitter {
 			viewItem = this.heightMap[i];
 
 			if (!viewItem) {
+				console.error('view item doesnt exist');
 				return;
 			}