diff --git a/ChangeLog b/ChangeLog
index ea182c48bd..ae2fe7bd4b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -15,6 +15,8 @@ Bugfix
    * Remove a duplicate #include in a sample program. Fixed by Masashi Honma #2326.
    * Remove the mbedtls namespacing from the header file, to fix a "file not found"
      build error. Fixed by Haijun Gu #2319.
+   * Fix signed-to-unsigned integer conversion warning
+     in X.509 module. Fixes #2212.
 
 Changes
    * Include configuration file in all header files that use configuration,
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 024faf4e7b..32aca012b5 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -368,7 +368,7 @@ static void x509_crt_verify_chain_reset(
     for( i = 0; i < MBEDTLS_X509_MAX_VERIFY_CHAIN_SIZE; i++ )
     {
         ver_chain->items[i].crt = NULL;
-        ver_chain->items[i].flags = -1;
+        ver_chain->items[i].flags = (uint32_t) -1;
     }
 
     ver_chain->len = 0;