diff --git a/third_party/bigint/BigIntegerUtils.cc b/third_party/bigint/BigIntegerUtils.cc
index f48334d..eba218d 100644
--- a/third_party/bigint/BigIntegerUtils.cc
+++ b/third_party/bigint/BigIntegerUtils.cc
@@ -4,6 +4,8 @@
 
 // Original code by Matt McCutchen, see the LICENSE file.
 
+#include <ostream>
+
 #include "BigIntegerUtils.hh"
 #include "BigUnsignedInABase.hh"
 
diff --git a/third_party/bigint/BigIntegerUtils.hh b/third_party/bigint/BigIntegerUtils.hh
index 999cdd2..7da7e9f 100644
--- a/third_party/bigint/BigIntegerUtils.hh
+++ b/third_party/bigint/BigIntegerUtils.hh
@@ -9,7 +9,7 @@
 
 #include "BigInteger.hh"
 #include <string>
-#include <ostream>
+#include <iosfwd>
 
 /* This file provides:
  * - Convenient std::string <-> BigUnsigned/BigInteger conversion routines