diff --git a/src/qt/CMakeLists.txt b/src/qt/CMakeLists.txt
--- a/src/qt/CMakeLists.txt
+++ b/src/qt/CMakeLists.txt
@@ -252,6 +252,12 @@
 		target_disable_clang_tidy(bitcoin-qt-protobuf)
 	endif()
 
+	# Message::ByteSize() is deprecated and replaced by ByteSizeLong() since
+	# protobuf 3.1.
+	if(Protobuf_VERSION GREATER_EQUAL "3.1.0")
+		target_compile_definitions(bitcoin-qt-base PRIVATE USE_PROTOBUF_MESSAGE_BYTESIZELONG)
+	endif()
+
 	# OpenSSL functionality
 	include(BrewHelper)
 	find_brew_prefix(OPENSSL_ROOT_DIR openssl)
diff --git a/src/qt/paymentserver.cpp b/src/qt/paymentserver.cpp
--- a/src/qt/paymentserver.cpp
+++ b/src/qt/paymentserver.cpp
@@ -766,11 +766,16 @@
                       "key, refund_to not set";
     }
 
-    int length = payment.ByteSize();
+    QVariant length;
+#ifdef USE_PROTOBUF_MESSAGE_BYTESIZELONG
+    length.setValue(payment.ByteSizeLong());
+#else
+    length.setValue(payment.ByteSize());
+#endif
 
     netRequest.setHeader(QNetworkRequest::ContentLengthHeader, length);
-    QByteArray serData(length, '\0');
-    if (payment.SerializeToArray(serData.data(), length)) {
+    QByteArray serData(length.toInt(), '\0');
+    if (payment.SerializeToArray(serData.data(), length.toInt())) {
         netManager->post(netRequest, serData);
     } else {
         // This should never happen, either.