diff --git a/library/asn1write.c b/library/asn1write.c
index 2e7c4a96f..6e806151d 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -77,7 +77,7 @@ int mbedtls_asn1_write_tag(unsigned char **p, const unsigned char *start, unsign
 
 static int mbedtls_write_len_and_tag(unsigned char **p,
                                      const unsigned char *start,
-                                     int len,
+                                     size_t len,
                                      unsigned char tag)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;