diff --git a/builder.cpp b/builder.cpp
index f4ba922..69514aa 100644
--- a/builder.cpp
+++ b/builder.cpp
@@ -14,6 +14,7 @@
 #include <nlohmann/json.hpp>
 #include <fstream>
 #include <future>
+#include <mutex>
 
 using std::string;
 using namespace fmt;
@@ -32,13 +33,15 @@ Builder::Builder(GUI &g, GameEngine &engine)
   config["build_cmd"].template get_to<string>(build_cmd);
 }
 
-FILE *start_command(string &build_cmd) {
+FILE *Builder::start_command(string &build_cmd) {
+  std::lock_guard<std::mutex> lock(fsm_mutex);
   FILE *build_out = popen(build_cmd.c_str(), "r");
   dbc::check(build_out != nullptr, "Failed to run command.");
   return build_out;
 }
 
-string read_line(FILE *build_out, bool &done_out) {
+string Builder::read_line(FILE *build_out, bool &done_out) {
+  std::lock_guard<std::mutex> lock(fsm_mutex);
   char buffer[BUF_MAX];
   char *res = fgets(buffer, BUF_MAX, build_out);
   done_out = res == nullptr;
diff --git a/builder.hpp b/builder.hpp
index 7b3ae5c..c8341b3 100644
--- a/builder.hpp
+++ b/builder.hpp
@@ -42,6 +42,7 @@ class Builder : DeadSimpleFSM<BuildState, BuildEvent> {
   string line = "";
   std::future<FILE *> build_fut;
   std::future<string> read_fut;
+  std::mutex fsm_mutex;
   git_repository* repo = nullptr;
 
   public:
@@ -49,6 +50,8 @@ class Builder : DeadSimpleFSM<BuildState, BuildEvent> {
   Builder(GUI &g, GameEngine &engine);
 
   MatchResult parse_line(const string &line);
+  string read_line(FILE *build_out, bool &done_out);
+  FILE *start_command(string &build_cmd);
 
   void event(BuildEvent ev) override {
     try {
diff --git a/game_engine.cpp b/game_engine.cpp
index 3614526..460b2a5 100644
--- a/game_engine.cpp
+++ b/game_engine.cpp
@@ -64,7 +64,7 @@ void GameEngine::idle(GameEvent ev) {
 void GameEngine::in_round(GameEvent ev, string &hit_type) {
   switch(ev) {
     case GameEvent::HIT:
-      hit(hit_type); // FIXME: bring back error type
+      hit(hit_type);
       if(is_dead()) {
         state(GameState::DEAD);
       } else {