diff --git a/lib/paymentwall/pingback.js b/lib/paymentwall/pingback.js
index 7dc65e2..8d300a0 100644
--- a/lib/paymentwall/pingback.js
+++ b/lib/paymentwall/pingback.js
@@ -25,7 +25,7 @@ util._extend(Pingback.prototype, {
 
 		if (this.isParametersValid()) {
 			if (this.isIpAddressValid() || skipIpWhitelistCheck) {
-				if (this.isSignatureValid()) {
+				if (this.isSignatureValid.bind(this)) {
 					validated = true;
 				} else {
 					this.appendToErrors('Wrong signature');
@@ -240,4 +240,4 @@ util._extend(Pingback.prototype, {
 	}
 });
 
-module.exports = Pingback;
\ No newline at end of file
+module.exports = Pingback;