diff --git a/base/error_state.h b/base/error_state.h
index 9864f6a..079fed9 100644
--- a/base/error_state.h
+++ b/base/error_state.h
@@ -11,6 +11,11 @@ namespace base {
 	};
 
 	error_state combine_errors(error_state lhs, error_state rhs);
+
+	inline error_state &operator <<(error_state &err, error_state rhs) {
+		err = combine_errors(err, rhs);
+		return err;
+	}
 }
 
 //----------------------------------------------------------------
diff --git a/unit-tests/Makefile.in b/unit-tests/Makefile.in
index d57f4ce..9307e5f 100644
--- a/unit-tests/Makefile.in
+++ b/unit-tests/Makefile.in
@@ -57,6 +57,7 @@ TEST_SOURCE=\
 	unit-tests/cache_superblock_t.cc \
 	unit-tests/damage_tracker_t.cc \
 	unit-tests/endian_t.cc \
+	unit-tests/error_state_t.cc \
 	unit-tests/rmap_visitor_t.cc \
 	unit-tests/run_set_t.cc \
 	unit-tests/space_map_t.cc \
diff --git a/unit-tests/error_state_t.cc b/unit-tests/error_state_t.cc
new file mode 100644
index 0000000..55e096c
--- /dev/null
+++ b/unit-tests/error_state_t.cc
@@ -0,0 +1,31 @@
+#include "gmock/gmock.h"
+#include "base/error_state.h"
+
+using namespace base;
+using namespace std;
+using namespace testing;
+
+//----------------------------------------------------------------
+
+TEST(ErrorStateTests, stream_operator)
+{
+	{
+		error_state err = NO_ERROR;
+		err << NO_ERROR << FATAL;
+		ASSERT_THAT(err, Eq(FATAL));
+	}
+
+	{
+		error_state err = NO_ERROR;
+		err << NO_ERROR << NON_FATAL;
+		ASSERT_THAT(err, Eq(NON_FATAL));
+	}
+
+	{
+		error_state err = NO_ERROR;
+		err << NO_ERROR << FATAL << NO_ERROR << NON_FATAL;
+		ASSERT_THAT(err, Eq(FATAL));
+	}
+}
+
+//----------------------------------------------------------------