diff --git a/library/sha1.c b/library/sha1.c
index 8c77cba..2ccf2a2 100644
--- a/library/sha1.c
+++ b/library/sha1.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;
 }
 
 /*