diff --git a/mongoose.c b/mongoose.c
index b2342b79..2730aff7 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -921,7 +921,7 @@ int cs_base64_decode(const unsigned char *s, int len, char *dst) {
 #if !(_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L) &&    \
         !(__DARWIN_C_LEVEL >= 200809L) && !defined(RTOS_SDK) || \
     defined(_WIN32)
-int strnlen(const char *s, size_t maxlen) {
+size_t strnlen(const char *s, size_t maxlen) {
   size_t l = 0;
   for (; l < maxlen && s[l] != '\0'; l++) {
   }
diff --git a/mongoose.h b/mongoose.h
index 3448f090..c57dba26 100644
--- a/mongoose.h
+++ b/mongoose.h
@@ -483,7 +483,7 @@ int c_vsnprintf(char *buf, size_t buf_size, const char *format, va_list ap);
 #if !(_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L) &&    \
         !(__DARWIN_C_LEVEL >= 200809L) && !defined(RTOS_SDK) || \
     defined(_WIN32)
-int strnlen(const char *s, size_t maxlen);
+size_t strnlen(const char *s, size_t maxlen);
 #endif
 
 #ifdef __cplusplus