diff --git a/library/x509_crt.c b/library/x509_crt.c
index 00f3107..d72e2fb 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -679,8 +679,8 @@
     }
 
     if (*p != end) {
-        return MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
-               MBEDTLS_ERR_ASN1_LENGTH_MISMATCH;
+        return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                                 MBEDTLS_ERR_ASN1_LENGTH_MISMATCH);
     }
 
     return 0;
diff --git a/tests/scripts/check_files.py b/tests/scripts/check_files.py
index e937202..87326e8 100755
--- a/tests/scripts/check_files.py
+++ b/tests/scripts/check_files.py
@@ -446,6 +446,25 @@
         return False
 
 
+class ErrorAddIssueTracker(LineIssueTracker):
+    """Signal direct additions of error codes.
+
+    Adding a low-level error code with a high-level error code is deprecated
+    and should use MBEDTLS_ERROR_ADD.
+    """
+
+    heading = "Direct addition of error codes"
+
+    _ERR_PLUS_RE = re.compile(br'MBEDTLS_ERR_\w+ *\+|'
+                              br'\+ *MBEDTLS_ERR_')
+    _EXCLUDE_RE = re.compile(br' *case ')
+
+    def issue_with_line(self, line, filepath, line_number):
+        if self._ERR_PLUS_RE.search(line) and not self._EXCLUDE_RE.match(line):
+            return True
+        return False
+
+
 class IntegrityChecker:
     """Sanity-check files under the current directory."""
 
@@ -467,6 +486,7 @@
             TabIssueTracker(),
             MergeArtifactIssueTracker(),
             LicenseIssueTracker(),
+            ErrorAddIssueTracker(),
         ]
 
     def setup_logger(self, log_file, level=logging.INFO):