diff --git a/library/cipher_wrap.c b/library/cipher_wrap.c
index 79daaf9..f09823a 100644
--- a/library/cipher_wrap.c
+++ b/library/cipher_wrap.c
@@ -786,7 +786,11 @@
 static int arc4_setkey_wrap( void *ctx, const unsigned char *key,
                              unsigned int key_length )
 {
-    arc4_setup( (arc4_context *) ctx, key, key_length );
+    /* we get key_length in bits, arc4 expects it in bytes */
+    if( key_length % 8 != 0)
+        return( POLARSSL_ERR_CIPHER_BAD_INPUT_DATA );
+
+    arc4_setup( (arc4_context *) ctx, key, key_length / 8 );
     return( 0 );
 }