diff --git a/lib/idn.c b/lib/idn.c
index 083be1d53f..ae781d3ca6 100644
--- a/lib/idn.c
+++ b/lib/idn.c
@@ -185,6 +185,9 @@ static CURLcode win32_idn_to_ascii(const char *in, char **out)
   wchar_t in_w[IDN_MAX_LENGTH];
   int in_w_len;
   *out = NULL;
+  /* Returned in_w_len includes the null-terminator, which then gets
+     preserved across the calls that follow, ending up terminating
+     the buffer returned to the caller. */
   in_w_len = MultiByteToWideChar(CP_UTF8, 0, in, -1, in_w, IDN_MAX_LENGTH);
   if(in_w_len) {
     wchar_t punycode[IDN_MAX_LENGTH];
@@ -208,6 +211,9 @@ static CURLcode win32_ascii_to_idn(const char *in, char **out)
   wchar_t in_w[IDN_MAX_LENGTH];
   int in_w_len;
   *out = NULL;
+  /* Returned in_w_len includes the null-terminator, which then gets
+     preserved across the calls that follow, ending up terminating
+     the buffer returned to the caller. */
   in_w_len = MultiByteToWideChar(CP_UTF8, 0, in, -1, in_w, IDN_MAX_LENGTH);
   if(in_w_len) {
     WCHAR idn[IDN_MAX_LENGTH]; /* stores a UTF-16 string */