diff --git a/include/mbedtls/oid.h b/include/mbedtls/oid.h
index da3d70ed5..68ae72317 100644
--- a/include/mbedtls/oid.h
+++ b/include/mbedtls/oid.h
@@ -479,8 +479,9 @@ int mbedtls_oid_get_numeric_string(char *buf, size_t size, const mbedtls_asn1_bu
  *                  heap. It must be freed by the caller using mbedtls_free().
  *
  * \param oid       #mbedtls_asn1_buf to populate with the DER-encoded OID
- * \param oid_str   string representation of the OID to parse
- * \param size      length of the OID string
+ * \param oid_str   string representation of the OID to parse, not
+ *                  NUL-terminated
+ * \param size      length of the OID string, not including any NUL terminator
  *
  * \return          0 if successful
  * \return          #MBEDTLS_ERR_ASN1_INVALID_DATA if \p oid_str does not
diff --git a/library/oid.c b/library/oid.c
index 87e5d892f..312a6375b 100644
--- a/library/oid.c
+++ b/library/oid.c
@@ -960,7 +960,7 @@ int mbedtls_oid_from_numeric_string(mbedtls_asn1_buf *oid,
     size_t encoded_len;
     unsigned char *minimum_mem;
 
-    for (size_t i = 0; (i < size) && (oid_str[i] != '\0'); i++) {
+    for (size_t i = 0; i < size; i++) {
         if (oid_str[i] == '.') {
             num_dots++;
         }
@@ -1003,7 +1003,7 @@ int mbedtls_oid_from_numeric_string(mbedtls_asn1_buf *oid,
         ret = MBEDTLS_ERR_ASN1_INVALID_DATA;
         goto error;
     }
-    if (str_ptr < str_bound && *str_ptr != '\0') {
+    if (str_ptr < str_bound) {
         if (*str_ptr == '.') {
             str_ptr++;
         } else {
@@ -1022,12 +1022,12 @@ int mbedtls_oid_from_numeric_string(mbedtls_asn1_buf *oid,
         goto error;
     }
 
-    while (str_ptr < str_bound && *str_ptr != '\0') {
+    while (str_ptr < str_bound) {
         ret = oid_parse_number(&val, &str_ptr, str_bound);
         if (ret != 0) {
             goto error;
         }
-        if (str_ptr < str_bound && *str_ptr != '\0') {
+        if (str_ptr < str_bound) {
             if (*str_ptr == '.') {
                 str_ptr++;
             } else {