diff --git a/dlls/wininet/internet.c b/dlls/wininet/internet.c
index 6166ff8673c..ea1d294683e 100644
--- a/dlls/wininet/internet.c
+++ b/dlls/wininet/internet.c
@@ -2478,6 +2478,12 @@ BOOL WINAPI InternetTimeFromSystemTimeA( const SYSTEMTIME* time, DWORD format, L
 
     TRACE( "%p 0x%08x %p 0x%08x\n", time, format, string, size );
 
+    if (size < INTERNET_RFC1123_BUFSIZE * sizeof(*string))
+    {
+        SetLastError(ERROR_INSUFFICIENT_BUFFER);
+        return FALSE;
+    }
+
     ret = InternetTimeFromSystemTimeW( time, format, stringW, sizeof(stringW) );
     if (ret) WideCharToMultiByte( CP_ACP, 0, stringW, -1, string, size, NULL, NULL );
 
@@ -2495,11 +2501,15 @@ BOOL WINAPI InternetTimeFromSystemTimeW( const SYSTEMTIME* time, DWORD format, L
 
     TRACE( "%p 0x%08x %p 0x%08x\n", time, format, string, size );
 
-    if (!time || !string) return FALSE;
-
-    if (format != INTERNET_RFC1123_FORMAT || size < INTERNET_RFC1123_BUFSIZE * sizeof(WCHAR))
+    if (!time || !string || format != INTERNET_RFC1123_FORMAT)
         return FALSE;
 
+    if (size < INTERNET_RFC1123_BUFSIZE * sizeof(*string))
+    {
+        SetLastError(ERROR_INSUFFICIENT_BUFFER);
+        return FALSE;
+    }
+
     sprintfW( string, date,
               WININET_wkday[time->wDayOfWeek],
               time->wDay,
diff --git a/dlls/wininet/tests/http.c b/dlls/wininet/tests/http.c
index db4f075c370..bfdcf80b7c6 100644
--- a/dlls/wininet/tests/http.c
+++ b/dlls/wininet/tests/http.c
@@ -800,12 +800,10 @@ static void InternetTimeFromSystemTimeA_test(void)
     SetLastError(0xdeadbeef);
     ret = pInternetTimeFromSystemTimeA( &time, INTERNET_RFC1123_FORMAT, string, 0 );
     error = GetLastError();
-    todo_wine {
     ok( !ret, "InternetTimeFromSystemTimeA should have returned FALSE\n" );
     ok( error == ERROR_INSUFFICIENT_BUFFER,
         "InternetTimeFromSystemTimeA failed with ERROR_INSUFFICIENT_BUFFER instead of %u\n",
         error );
-    }
 }
 
 static void InternetTimeFromSystemTimeW_test(void)
@@ -827,7 +825,6 @@ static void InternetTimeFromSystemTimeW_test(void)
     ret = pInternetTimeFromSystemTimeW( &time, INTERNET_RFC1123_FORMAT, string, sizeof(string)/sizeof(string[0]) );
     error = GetLastError();
     ok( !ret, "InternetTimeFromSystemTimeW should have returned FALSE\n" );
-    todo_wine
     ok( error == ERROR_INSUFFICIENT_BUFFER,
         "InternetTimeFromSystemTimeW failed with ERROR_INSUFFICIENT_BUFFER instead of %u\n",
         error );