diff --git a/src/strhelper.c b/src/strhelper.c
index a3d1dd6a..f1d0cef2 100644
--- a/src/strhelper.c
+++ b/src/strhelper.c
@@ -16,6 +16,7 @@
  * limitations under the License.
  */
 
+#include <ctype.h>
 #include <stddef.h>
 #include <string.h>
 #include "private/memory.h"
diff --git a/tools/check_headers/c_standard_library.yml b/tools/check_headers/c_standard_library.yml
index a8c6b558..691ee054 100644
--- a/tools/check_headers/c_standard_library.yml
+++ b/tools/check_headers/c_standard_library.yml
@@ -16,6 +16,7 @@
   - "stdlib.h"
   - "string.h"
   - "time.h"
+"tolower": "ctype.h"
 "true": "stdbool.h"
 "va_end": "stdarg.h"
 "va_list": "stdarg.h"