diff --git a/include/sta/Report.hh b/include/sta/Report.hh
index 11fe305c..6dd0ca47 100644
--- a/include/sta/Report.hh
+++ b/include/sta/Report.hh
@@ -19,6 +19,7 @@
 #include <stdio.h>
 #include <stdarg.h>
 #include <string>
+#include <mutex>
 #include "DisallowCopyAssign.hh"
 
 struct Tcl_Interp;
@@ -113,6 +114,7 @@ protected:
   char *buffer_;
   // Length of string in buffer.
   size_t buffer_length_;
+  std::mutex buffer_lock_;
 
 private:
   DISALLOW_COPY_AND_ASSIGN(Report);
diff --git a/util/Report.cc b/util/Report.cc
index 7c1617d5..0d8d07eb 100644
--- a/util/Report.cc
+++ b/util/Report.cc
@@ -88,6 +88,7 @@ Report::print(const string &str)
 void
 Report::vprint(const char *fmt, va_list args)
 {
+  std::unique_lock<std::mutex> lock(buffer_lock_);
   printToBuffer(fmt, args);
   printString(buffer_, buffer_length_);
 }
@@ -121,6 +122,7 @@ Report::printError(const char *buffer, size_t length)
 void
 Report::vprintError(const char *fmt, va_list args)
 {
+  std::unique_lock<std::mutex> lock(buffer_lock_);
   printToBuffer(fmt, args);
   printError(buffer_, buffer_length_);
 }