diff --git a/libs/libc/string/lib_strcmp.c b/libs/libc/string/lib_strcmp.c
index 2ac4f62619a..edb6d23c834 100644
--- a/libs/libc/string/lib_strcmp.c
+++ b/libs/libc/string/lib_strcmp.c
@@ -34,11 +34,14 @@
 #undef strcmp /* See mm/README.txt */
 int strcmp(FAR const char *cs, FAR const char *ct)
 {
-  register signed char result;
+  register int result;
   for (; ; )
     {
-      if ((result = *cs - *ct++) != 0 || !*cs++)
-      break;
+      if ((result = (unsigned char)*cs - (unsigned char)*ct++) != 0 ||
+          !*cs++)
+        {
+          break;
+        }
     }
 
   return result;
diff --git a/libs/libc/string/lib_strncmp.c b/libs/libc/string/lib_strncmp.c
index 117f1fddf88..48e2a2486aa 100644
--- a/libs/libc/string/lib_strncmp.c
+++ b/libs/libc/string/lib_strncmp.c
@@ -37,7 +37,8 @@ int strncmp(const char *cs, const char *ct, size_t nb)
   int result = 0;
   for (; nb > 0; nb--)
     {
-      if ((result = (int)*cs - (int)*ct++) != 0 || !*cs++)
+      if ((result = (unsigned char)*cs - (unsigned char)*ct++) != 0 ||
+          !*cs++)
         {
           break;
         }