diff --git a/library/gcm.c b/library/gcm.c
index fc03acd..d7acace 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -304,7 +304,7 @@
     int i = 0;
     unsigned char lo, hi, rem;
     uint64_t u64z[2];
-    const uint64_t *pu64z = 0;
+    const uint64_t *pu64z = NULL;
     uint8_t *u8z = (uint8_t *) u64z;
 
     lo = x[15] & 0xf;