diff --git a/objectivec/GPBUnknownField.m b/objectivec/GPBUnknownField.m
index 9d5c97f382..7fa8cade2d 100644
--- a/objectivec/GPBUnknownField.m
+++ b/objectivec/GPBUnknownField.m
@@ -32,6 +32,7 @@
 
 #import "GPBArray.h"
 #import "GPBCodedOutputStream_PackagePrivate.h"
+#import "GPBUnknownFieldSet.h"
 
 @implementation GPBUnknownField {
  @protected