diff --git a/src/bogofilter-smtpd.cpp b/src/bogofilter-smtpd.cpp
index 5e65366..cef1bb7 100644
--- a/src/bogofilter-smtpd.cpp
+++ b/src/bogofilter-smtpd.cpp
@@ -55,11 +55,11 @@ void on_data_line(protocol::protocol_t& protocol, protocol::message_t message) {
             protocol.submit_message(session_key, session_token, session.message.header, session.message.body);
             FILE* f = popen("bogofilter -l", "w");
             for(string& line : session.message.header) {
-                fprintf(f, "%s\n", line);
+                fprintf(f, "%s\n", line.c_str());
             }
             fwrite("\n", 1, 1, f);
             for(string& line : session.message.body) {
-                fprintf(f, "%s\n", line);
+                fprintf(f, "%s\n", line.c_str());
             }
             int status = pclose(f);
             session.status = WEXITSTATUS(status);