diff --git a/tests/.#extensions.cpp b/tests/.#extensions.cpp
new file mode 120000
index 0000000..d422a10
--- /dev/null
+++ b/tests/.#extensions.cpp
@@ -0,0 +1 @@
+simon@archer.2493:1509089313
\ No newline at end of file
diff --git a/tests/extensions.cpp b/tests/extensions.cpp
index bc619a9..62cab82 100644
--- a/tests/extensions.cpp
+++ b/tests/extensions.cpp
@@ -132,6 +132,67 @@ TEST_CASE("Map extensions", "[extensions.map]") {
   }
 }
 
+TEST_CASE("Map error extensions", "[extensions.map_error]") {
+  auto mul2 = [](auto a) { return a * 2; };
+  auto ret_void = [](auto a) {};
+
+  {
+    tl::expected<int, int> e = 21;
+    auto ret = e.map_error(mul2);
+    REQUIRE(ret);
+    REQUIRE(*ret == 21);
+  }
+
+  {
+    const tl::expected<int, int> e = 21;
+    auto ret = e.map_error(mul2);
+    REQUIRE(ret);
+    REQUIRE(*ret == 21);
+  }
+
+  {
+    tl::expected<int, int> e = 21;
+    auto ret = std::move(e).map_error(mul2);
+    REQUIRE(ret);
+    REQUIRE(*ret == 21);
+  }
+
+  {
+    const tl::expected<int, int> e = 21;
+    auto ret = std::move(e).map_error(mul2);
+    REQUIRE(ret);
+    REQUIRE(*ret == 21);
+  }
+
+  {
+    tl::expected<int, int> e(tl::unexpect, 21);
+    auto ret = e.map_error(mul2);
+    REQUIRE(!ret);
+    REQUIRE(ret.error() == 42);
+  }
+
+  {
+    const tl::expected<int, int> e(tl::unexpect, 21);
+    auto ret = e.map_error(mul2);
+    REQUIRE(!ret);
+    REQUIRE(ret.error() == 42);
+  }
+
+  {
+    tl::expected<int, int> e(tl::unexpect, 21);
+    auto ret = std::move(e).map_error(mul2);
+    REQUIRE(!ret);
+    REQUIRE(ret.error() == 42);
+  }
+
+  {
+    const tl::expected<int, int> e(tl::unexpect, 21);
+    auto ret = std::move(e).map_error(mul2);
+    REQUIRE(!ret);
+    REQUIRE(ret.error() == 42);
+  }
+}
+
 TEST_CASE("And then extensions", "[extensions.and_then]") {
   auto succeed = [](auto a) { return tl::expected<int, int>(21 * 2); };
   auto fail = [](auto a) { return tl::expected<int, int>(tl::unexpect, 17); };
@@ -168,7 +229,7 @@ TEST_CASE("And then extensions", "[extensions.and_then]") {
     tl::expected<int, int> e = 21;
     auto ret = e.and_then(fail);
     REQUIRE(!ret);
-    REQUIRE(ret.error()  == 17);
+    REQUIRE(ret.error() == 17);
   }
 
   {