diff --git a/std/os/windows.zig b/std/os/windows.zig
index ac76e8f58f6aeeaaa2d698bfc5b3198b2275e3ff..4c6bfe70d500cffba5bf845fa728dc20ac8caa16 100644
--- a/std/os/windows.zig
+++ b/std/os/windows.zig
@@ -138,10 +138,19 @@ pub const RtlGenRandomError = error{Unexpected};
 /// https://github.com/rust-lang-nursery/rand/issues/111
 /// https://bugzilla.mozilla.org/show_bug.cgi?id=504270
 pub fn RtlGenRandom(output: []u8) RtlGenRandomError!void {
-    if (advapi32.RtlGenRandom(output.ptr, output.len) == 0) {
-        switch (kernel32.GetLastError()) {
-            else => |err| return unexpectedError(err),
+    var total_read: usize = 0;
+    var buff: []u8 = output[0..];
+    const max_read_size: ULONG = maxInt(ULONG);
+
+    while (total_read < output.len) {
+        const to_read: ULONG = math.min(buff.len, max_read_size);
+
+        if (advapi32.RtlGenRandom(buff.ptr, to_read) == 0) {
+            return unexpectedError(kernel32.GetLastError());
         }
+
+        total_read += to_read;
+        buff = buff[to_read..];
     }
 }
 
diff --git a/std/os/windows/advapi32.zig b/std/os/windows/advapi32.zig
index 165a2c10a3219707424d68cfb16bbd4f59d88294..940f10994cb20a18267e0484803e9faaacd3c6a5 100644
--- a/std/os/windows/advapi32.zig
+++ b/std/os/windows/advapi32.zig
@@ -19,5 +19,5 @@ pub extern "advapi32" stdcallcc fn RegQueryValueExW(
 
 // RtlGenRandom is known as SystemFunction036 under advapi32
 // http://msdn.microsoft.com/en-us/library/windows/desktop/aa387694.aspx */
-pub extern "advapi32" stdcallcc fn SystemFunction036(output: [*]u8, length: usize) BOOL;
+pub extern "advapi32" stdcallcc fn SystemFunction036(output: [*]u8, length: ULONG) BOOL;
 pub const RtlGenRandom = SystemFunction036;