diff --git a/src/kits/network/libnetapi/NetBuffer.cpp b/src/kits/network/libnetapi/NetBuffer.cpp
index 5bc5869696..bd05a68fd1 100644
--- a/src/kits/network/libnetapi/NetBuffer.cpp
+++ b/src/kits/network/libnetapi/NetBuffer.cpp
@@ -202,7 +202,11 @@ BNetBuffer::AppendMessage(const BMessage& data)
 {
 	char stackFlattenedData[STACK_BUFFER_SIZE];
 
-	size_t dataSize = data.FlattenedSize();
+	ssize_t dataSize = data.FlattenedSize();
+
+	if (dataSize < B_OK)
+		return dataSize;
+
 	if (dataSize == 0)
 		return B_ERROR;