diff --git a/Signature.class.php b/Signature.class.php
index 57e32e9..d80384d 100644
--- a/Signature.class.php
+++ b/Signature.class.php
@@ -255,7 +255,7 @@ public static function recoverPublicKey($R, $S, $hash, $recoveryFlags){
 		// Precalculate (p + 1) / 4 where p is the field order
 		// $p_over_four is GMP
 		static $p_over_four; // XXX just assuming only one curve/prime will be used
-		if (!$p_over_four) {
+		if (null === $p_over_four) {
 			$p_over_four = gmp_div(gmp_add($p, 1), 4);
 		}
 
@@ -412,4 +412,4 @@ public static function checkDerSignature($pubKey, $signature, $hash)
 
 }
 
-?>
\ No newline at end of file
+?>