diff --git a/src/export/formatter.spec.ts b/src/export/formatter.spec.ts
index bd6c9b7160..7ee05fa5d1 100644
--- a/src/export/formatter.spec.ts
+++ b/src/export/formatter.spec.ts
@@ -1,7 +1,8 @@
 import { assert, expect } from "chai";
+import * as sinon from "sinon";
 
 import { Formatter } from "export/formatter";
-import * as file from "file";
+import { Paragraph, TextRun } from "file";
 import { CoreProperties } from "file/core-properties";
 import { Attributes } from "file/xml-components";
 
@@ -14,22 +15,22 @@ describe("Formatter", () => {
 
     describe("#format()", () => {
         it("should format simple paragraph", () => {
-            const paragraph = new file.Paragraph("");
+            const paragraph = new Paragraph("");
             const newJson = formatter.format(paragraph);
             assert.isDefined(newJson["w:p"]);
         });
 
         it("should remove xmlKeys", () => {
-            const paragraph = new file.Paragraph("");
+            const paragraph = new Paragraph("");
             const newJson = formatter.format(paragraph);
             const stringifiedJson = JSON.stringify(newJson);
             assert(stringifiedJson.indexOf("xmlKeys") < 0);
         });
 
         it("should format simple paragraph with bold text", () => {
-            const paragraph = new file.Paragraph("");
+            const paragraph = new Paragraph("");
             paragraph.addRun(
-                new file.TextRun({
+                new TextRun({
                     text: "test",
                     bold: true,
                 }),
@@ -63,7 +64,7 @@ describe("Formatter", () => {
         });
 
         it("should should change 'p' tag into 'w:p' tag", () => {
-            const paragraph = new file.Paragraph("");
+            const paragraph = new Paragraph("");
             const newJson = formatter.format(paragraph);
             assert.isDefined(newJson["w:p"]);
         });
@@ -76,5 +77,13 @@ describe("Formatter", () => {
             const newJson = formatter.format(properties);
             assert.isDefined(newJson["cp:coreProperties"]);
         });
+
+        it("should call the prep method only once", () => {
+            const paragraph = new Paragraph("");
+            const spy = sinon.spy(paragraph, "prepForXml");
+
+            formatter.format(paragraph);
+            expect(spy.calledOnce).to.equal(true);
+        });
     });
 });