diff --git a/library/entropy_poll.c b/library/entropy_poll.c
index b5d9f78..eec8ec4 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -59,7 +59,7 @@
         return POLARSSL_ERR_ENTROPY_SOURCE_FAILED;
     }
 
-    if( CryptGenRandom( provider, len, output ) == FALSE )
+    if( CryptGenRandom( provider, (DWORD) len, output ) == FALSE )
         return POLARSSL_ERR_ENTROPY_SOURCE_FAILED;
 
     CryptReleaseContext( provider, 0 );