diff --git a/src/cpp/ripple/BigNum64.h b/src/cpp/ripple/BigNum64.h
index 0a052dda6..9aac72625 100644
--- a/src/cpp/ripple/BigNum64.h
+++ b/src/cpp/ripple/BigNum64.h
@@ -1,4 +1,7 @@
 
+#ifndef BIGNUM64_H
+#define BIGNUM64_H
+
 // Support 64-bit word operations on 32-bit platforms
 
 static int BN_add_word64(BIGNUM *a, uint64 w)
@@ -26,3 +29,5 @@ static uint64 BN_div_word64(BIGNUM *a, uint64 w)
 	CAutoBN_CTX ctx;
 	return (BN_div(a, NULL, a, &bn, ctx) == 1) ? 0 : ((uint64)-1);
 }
+
+#endif