diff --git a/src/document/doc-debug.js b/src/document/doc-debug.js
index 21400e0b0..6d7050929 100644
--- a/src/document/doc-debug.js
+++ b/src/document/doc-debug.js
@@ -3,6 +3,10 @@
 const { isConcat, getDocParts } = require("./doc-utils");
 
 function flattenDoc(doc) {
+  if (!doc) {
+    return "";
+  }
+
   if (isConcat(doc)) {
     const res = [];
     for (const part of getDocParts(doc)) {
@@ -22,8 +26,8 @@ function flattenDoc(doc) {
   if (doc.type === "if-break") {
     return {
       ...doc,
-      breakContents: doc.breakContents ? flattenDoc(doc.breakContents) : null,
-      flatContents: doc.flatContents ? flattenDoc(doc.flatContents) : null,
+      breakContents: flattenDoc(doc.breakContents),
+      flatContents: flattenDoc(doc.flatContents),
     };
   }
 
@@ -31,9 +35,7 @@ function flattenDoc(doc) {
     return {
       ...doc,
       contents: flattenDoc(doc.contents),
-      expandedStates: doc.expandedStates
-        ? doc.expandedStates.map(flattenDoc)
-        : doc.expandedStates,
+      expandedStates: doc.expandedStates && doc.expandedStates.map(flattenDoc),
     };
   }
 
diff --git a/tests_integration/__tests__/debug-api.js b/tests_integration/__tests__/debug-api.js
index 776f8e070..3cdcab017 100644
--- a/tests_integration/__tests__/debug-api.js
+++ b/tests_integration/__tests__/debug-api.js
@@ -63,6 +63,7 @@ describe("API", () => {
       hardline,
       literalline,
       fill,
+      ifBreak,
       indentIfBreak,
       group,
       line,
@@ -91,5 +92,9 @@ describe("API", () => {
     expect(formatDoc(label("foo", group(["1", line, "2"])))).toBe(
       'label("foo", group(["1", line, "2"]))'
     );
+
+    expect(formatDoc([ifBreak("a", "b"), ifBreak("a"), ifBreak("", "b")])).toBe(
+      '[ifBreak("a", "b"), ifBreak("a"), ifBreak("", "b")]'
+    );
   });
 });