diff --git a/javascript/TreeSelectorField.js b/javascript/TreeSelectorField.js
index dce02f6f8..3dd28aa50 100755
--- a/javascript/TreeSelectorField.js
+++ b/javascript/TreeSelectorField.js
@@ -99,7 +99,7 @@ TreeDropdownField.prototype = {
 			}
 			
 			// This iframe stretching doesn't work with the greybox
-			if(this.iframeObj.id == 'GB_frame') return;
+			if(this.iframeObj && this.iframeObj.id == 'GB_frame') return;
 			
 			var desiredHeight = Position.cumulativeOffset(this.itemTree)[1] + this.itemTree.offsetHeight + 2;
 			if(this.iframeObj && desiredHeight > this.iframeObj.offsetHeight) {