diff --git a/libraries/AP_Logger/AP_Logger.cpp b/libraries/AP_Logger/AP_Logger.cpp
index 697db9dd00..7c5cf2e352 100644
--- a/libraries/AP_Logger/AP_Logger.cpp
+++ b/libraries/AP_Logger/AP_Logger.cpp
@@ -855,6 +855,8 @@ void AP_Logger::assert_same_fmt_for_name(const AP_Logger::log_write_fmt *f,
 
 AP_Logger::log_write_fmt *AP_Logger::msg_fmt_for_name(const char *name, const char *labels, const char *units, const char *mults, const char *fmt)
 {
+    WITH_SEMAPHORE(log_write_fmts_sem);
+
     struct log_write_fmt *f;
     for (f = log_write_fmts; f; f=f->next) {
         if (f->name == name) { // ptr comparison
diff --git a/libraries/AP_Logger/AP_Logger.h b/libraries/AP_Logger/AP_Logger.h
index 6bc521e143..603097f0ac 100644
--- a/libraries/AP_Logger/AP_Logger.h
+++ b/libraries/AP_Logger/AP_Logger.h
@@ -413,6 +413,7 @@ private:
         const char *units;
         const char *mults;
     } *log_write_fmts;
+    HAL_Semaphore_Recursive log_write_fmts_sem;
 
     // return (possibly allocating) a log_write_fmt for a name
     struct log_write_fmt *msg_fmt_for_name(const char *name, const char *labels, const char *units, const char *mults, const char *fmt);