diff --git a/src/basic/log.h b/src/basic/log.h
index 0597bd1e84..2da6d5767d 100644
--- a/src/basic/log.h
+++ b/src/basic/log.h
@@ -35,9 +35,8 @@ typedef enum LogTarget{
  * used as a regular log level. */
 #define LOG_NULL (LOG_EMERG - 1)
 
-/* Note to readers: << and >> have lower precedence (are evaluated earlier) than & and | */
-#define SYNTHETIC_ERRNO(num)                (1 << 30 | (num))
-#define IS_SYNTHETIC_ERRNO(val)             ((val) >> 30 & 1)
+#define SYNTHETIC_ERRNO(num)                (abs(num) | (1 << 30))
+#define IS_SYNTHETIC_ERRNO(val)             (((val) >> 30) == 1)
 #define ERRNO_VALUE(val)                    (abs(val) & ~(1 << 30))
 
 /* The callback function to be invoked when syntax warnings are seen
diff --git a/src/test/test-log.c b/src/test/test-log.c
index 97eb5e0199..57cab63269 100644
--- a/src/test/test-log.c
+++ b/src/test/test-log.c
@@ -13,11 +13,6 @@
 #include "strv.h"
 #include "tests.h"
 
-assert_cc(IS_SYNTHETIC_ERRNO(SYNTHETIC_ERRNO(EINVAL)));
-assert_cc(!IS_SYNTHETIC_ERRNO(EINVAL));
-assert_cc(IS_SYNTHETIC_ERRNO(SYNTHETIC_ERRNO(0)));
-assert_cc(!IS_SYNTHETIC_ERRNO(0));
-
 #define X10(x) x x x x x x x x x x
 #define X100(x) X10(X10(x))
 #define X1000(x) X100(X10(x))
@@ -227,6 +222,15 @@ static void test_log_prefix(void) {
 int main(int argc, char* argv[]) {
         test_setup_logging(LOG_DEBUG);
 
+        ASSERT_TRUE(IS_SYNTHETIC_ERRNO(SYNTHETIC_ERRNO(EINVAL)));
+        ASSERT_TRUE(IS_SYNTHETIC_ERRNO(SYNTHETIC_ERRNO(-EINVAL)));
+        assert_cc(!IS_SYNTHETIC_ERRNO(EINVAL));
+        assert_cc(!IS_SYNTHETIC_ERRNO(-EINVAL));
+        ASSERT_TRUE(IS_SYNTHETIC_ERRNO(SYNTHETIC_ERRNO(0)));
+        assert_cc(!IS_SYNTHETIC_ERRNO(0));
+        ASSERT_EQ(ERRNO_VALUE(EINVAL), EINVAL);
+        ASSERT_EQ(ERRNO_VALUE(SYNTHETIC_ERRNO(-EINVAL)), EINVAL);
+
         test_assert_return_is_critical();
         test_file();