diff --git a/src/server/scriptVersionCache.ts b/src/server/scriptVersionCache.ts
index 022447e488e2e..35e39ce64f9ec 100644
--- a/src/server/scriptVersionCache.ts
+++ b/src/server/scriptVersionCache.ts
@@ -279,9 +279,9 @@ namespace ts.server {
         // REVIEW: can optimize by coalescing simple edits
         edit(pos: number, deleteLen: number, insertedText?: string) {
             this.changes.push(new TextChange(pos, deleteLen, insertedText));
-            if ((this.changes.length > ScriptVersionCache.changeNumberThreshold) ||
-                (deleteLen > ScriptVersionCache.changeLengthThreshold) ||
-                (insertedText && (insertedText.length > ScriptVersionCache.changeLengthThreshold))) {
+            if (this.changes.length > ScriptVersionCache.changeNumberThreshold ||
+                deleteLen > ScriptVersionCache.changeLengthThreshold ||
+                insertedText && insertedText.length > ScriptVersionCache.changeLengthThreshold) {
                 this.getSnapshot();
             }
         }
@@ -471,12 +471,9 @@ namespace ts.server {
             return !walkFns.done;
         }
 
-        edit(pos: number, deleteLength: number, newText?: string) {
-            function editFlat(source: string, s: number, dl: number, nt = "") {
-                return source.substring(0, s) + nt + source.substring(s + dl, source.length);
-            }
+        edit(pos: number, deleteLength: number, newText?: string): LineIndex {
             if (this.root.charCount() === 0) {
-                // TODO: assert deleteLength === 0
+                Debug.assert(deleteLength === 0); // Can't delete from empty document
                 if (newText !== undefined) {
                     this.load(LineIndex.linesFromText(newText).lines);
                     return this;
@@ -485,7 +482,8 @@ namespace ts.server {
             else {
                 let checkText: string;
                 if (this.checkEdits) {
-                    checkText = editFlat(this.getText(0, this.root.charCount()), pos, deleteLength, newText);
+                    const source = this.getText(0, this.root.charCount());
+                    checkText = source.slice(0, pos) + newText + source.slice(pos + deleteLength);
                 }
                 const walker = new EditWalker();
                 let suppressTrailingText = false;
@@ -513,14 +511,15 @@ namespace ts.server {
                         newText = newText ? newText + lineText : lineText;
                     }
                 }
-                if (pos < this.root.charCount()) {
-                    this.root.walk(pos, deleteLength, walker);
-                    walker.insertLines(newText, suppressTrailingText);
-                }
+
+                this.root.walk(pos, deleteLength, walker);
+                walker.insertLines(newText, suppressTrailingText);
+
                 if (this.checkEdits) {
-                    const updatedText = this.getText(0, this.root.charCount());
+                    const updatedText = walker.lineIndex.getText(0, walker.lineIndex.getLength());
                     Debug.assert(checkText === updatedText, "buffer edit mismatch");
                 }
+
                 return walker.lineIndex;
             }
         }