diff --git a/library/arc4.c b/library/arc4.c
index ff0e993..05b33d3 100644
--- a/library/arc4.c
+++ b/library/arc4.c
@@ -49,7 +49,7 @@
 
 /* Implementation that should never be optimized out by the compiler */
 static void mbedtls_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
+    volatile unsigned char *p = (unsigned char*)v; while( n-- ) *p++ = 0;
 }
 
 void mbedtls_arc4_init( mbedtls_arc4_context *ctx )