diff --git a/tests/unit/libutil/local.mk b/tests/unit/libutil/local.mk
index 6de96c0dc..bf18a9da7 100644
--- a/tests/unit/libutil/local.mk
+++ b/tests/unit/libutil/local.mk
@@ -18,6 +18,7 @@ libutil-tests_EXTRA_INCLUDES = \
 
 libutil-tests_CXXFLAGS += $(libutil-tests_EXTRA_INCLUDES)
 
-libutil-tests_LIBS = libutil-test-support libutil
+# libexpr is needed for exception serialization tests. sigh.
+libutil-tests_LIBS = libutil-test-support libutil libexpr
 
 libutil-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS)
diff --git a/tests/unit/libutil/serialise.cc b/tests/unit/libutil/serialise.cc
index 71d99b9a7..b3bbdd33e 100644
--- a/tests/unit/libutil/serialise.cc
+++ b/tests/unit/libutil/serialise.cc
@@ -1,5 +1,10 @@
 #include "serialise.hh"
+#include "error.hh"
+#include "fmt.hh"
+#include "libexpr/pos-table.hh"
+#include "ref.hh"
 #include "types.hh"
+#include "util.hh"
 
 #include <limits.h>
 #include <gtest/gtest.h>
@@ -53,4 +58,155 @@ TEST(ChainSource, move)
     ASSERT_EQ(buf, "33");
 }
 
+TEST(Sink, uint64_t)
+{
+    StringSink s;
+    s << 42;
+    ASSERT_EQ(s.s, std::string({42, 0, 0, 0, 0, 0, 0, 0}));
+}
+
+TEST(Sink, string_view)
+{
+    StringSink s;
+    s << "";
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            // length
+            0, 0, 0, 0, 0, 0, 0, 0,
+            // data (omitted)
+        })
+    );
+    // clang-format on
+
+    s = {};
+    s << "test";
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            // length
+            4, 0, 0, 0, 0, 0, 0, 0,
+            // data
+            't', 'e', 's', 't',
+            // padding
+            0, 0, 0, 0,
+        })
+    );
+    // clang-format on
+
+    s = {};
+    s << "longer string";
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            // length
+            13, 0, 0, 0, 0, 0, 0, 0,
+            // data
+            'l', 'o', 'n', 'g', 'e', 'r', ' ', 's', 't', 'r', 'i', 'n', 'g',
+            // padding
+            0, 0, 0,
+        })
+    );
+    // clang-format on
+}
+
+TEST(Sink, StringSet)
+{
+    StringSink s;
+    s << StringSet{};
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            // length
+            0, 0, 0, 0, 0, 0, 0, 0,
+            // data (omitted)
+        })
+    );
+    // clang-format on
+
+    s = {};
+    s << StringSet{"a", ""};
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            // length
+            2, 0, 0, 0, 0, 0, 0, 0,
+            // data ""
+            0, 0, 0, 0, 0, 0, 0, 0,
+            // data "a"
+            1, 0, 0, 0, 0, 0, 0, 0, 'a', 0, 0, 0, 0, 0, 0, 0,
+        })
+    );
+    // clang-format on
+}
+
+TEST(Sink, Strings)
+{
+    StringSink s;
+    s << Strings{};
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            // length
+            0, 0, 0, 0, 0, 0, 0, 0,
+            // data (omitted)
+        })
+    );
+    // clang-format on
+
+    s = {};
+    s << Strings{"a", ""};
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            // length
+            2, 0, 0, 0, 0, 0, 0, 0,
+            // data "a"
+            1, 0, 0, 0, 0, 0, 0, 0, 'a', 0, 0, 0, 0, 0, 0, 0,
+            // data ""
+            0, 0, 0, 0, 0, 0, 0, 0,
+        })
+    );
+    // clang-format on
+}
+
+TEST(Sink, Error)
+{
+    PosTable pt;
+    auto o = pt.addOrigin(Pos::String{make_ref<std::string>("test")}, 4);
+
+    StringSink s;
+    s << Error{{
+        .level = lvlInfo,
+        .msg = HintFmt("foo"),
+        .pos = pt[pt.add(o, 1)],
+        .traces = {{.pos = pt[pt.add(o, 2)], .hint = HintFmt("b %1%", "foo")}},
+    }};
+    // NOTE position of the error and all traces are ignored
+    // by the wire format
+    // clang-format off
+    ASSERT_EQ(
+        s.s,
+        std::string({
+            5, 0, 0, 0, 0, 0, 0, 0, 'E', 'r', 'r', 'o', 'r', 0, 0, 0,
+            3, 0, 0, 0, 0, 0, 0, 0,
+            5, 0, 0, 0, 0, 0, 0, 0, 'E', 'r', 'r', 'o', 'r', 0, 0, 0,
+            3, 0, 0, 0, 0, 0, 0, 0, 'f', 'o', 'o', 0, 0, 0, 0, 0,
+            0, 0, 0, 0, 0, 0, 0, 0,
+            1, 0, 0, 0, 0, 0, 0, 0,
+            0, 0, 0, 0, 0, 0, 0, 0,
+            16, 0, 0, 0, 0, 0, 0, 0,
+            'b', ' ', '\x1b', '[', '3', '5', ';', '1', 'm', 'f', 'o', 'o', '\x1b', '[', '0', 'm',
+        })
+    );
+    // clang-format on
+}
+
 }