diff --git a/include/fluidsynth/log.h b/include/fluidsynth/log.h
index eca44200..00d802f5 100644
--- a/include/fluidsynth/log.h
+++ b/include/fluidsynth/log.h
@@ -70,12 +70,12 @@ enum fluid_log_level
  * @param message Log message text
  * @param data User data pointer supplied to fluid_set_log_function().
  */
-typedef void (*fluid_log_function_t)(int level, char *message, void *data);
+typedef void (*fluid_log_function_t)(int level, const char *message, void *data);
 
 FLUIDSYNTH_API
 fluid_log_function_t fluid_set_log_function(int level, fluid_log_function_t fun, void *data);
 
-FLUIDSYNTH_API void fluid_default_log_function(int level, char *message, void *data);
+FLUIDSYNTH_API void fluid_default_log_function(int level, const char *message, void *data);
 
 FLUIDSYNTH_API int fluid_log(int level, const char *fmt, ...);
 
diff --git a/src/utils/fluid_sys.c b/src/utils/fluid_sys.c
index 30ffe3f4..15fce83f 100644
--- a/src/utils/fluid_sys.c
+++ b/src/utils/fluid_sys.c
@@ -118,7 +118,7 @@ fluid_set_log_function(int level, fluid_log_function_t fun, void *data)
  * @param data User supplied data (not used)
  */
 void
-fluid_default_log_function(int level, char *message, void *data)
+fluid_default_log_function(int level, const char *message, void *data)
 {
     FILE *out;