diff --git a/hiredis.c b/hiredis.c
index 4544c28..97ef78b 100644
--- a/hiredis.c
+++ b/hiredis.c
@@ -806,7 +806,7 @@ void *redisCommand(redisContext *c, const char *format, ...) {
  * Always returns NULL. In a non-blocking context this will never fail because
  * this function does not do any I/O. In a blocking context this function will
  * have no effect (a callback in a blocking context makes no sense). */
-void *redisCommandWithCallback(redisContext *c, redisCallbackFn *fn, void *privdata, const char *format, ...) {
+void *redisCommandWithCallback(redisContext *c, redisCallbackFn *fn, const void *privdata, const char *format, ...) {
     va_list ap;
     sds cmd;
     int status;
diff --git a/hiredis.h b/hiredis.h
index 72a4077..0dd32bd 100644
--- a/hiredis.h
+++ b/hiredis.h
@@ -76,10 +76,10 @@ struct redisContext; /* need forward declaration of redisContext */
 typedef void (redisContextCallback)(struct redisContext*, void*);
 
 /* Reply callback prototype and container */
-typedef void redisCallbackFn(struct redisContext*, redisReply*, void*);
+typedef void redisCallbackFn(struct redisContext*, redisReply*, const void*);
 typedef struct redisCallback {
     redisCallbackFn *fn;
-    void *privdata;
+    const void *privdata;
 } redisCallback;
 
 /* Context for a connection to Redis */
@@ -124,6 +124,6 @@ int redisGetReply(redisContext *c, void **reply);
 int redisProcessCallbacks(redisContext *c);
 
 void *redisCommand(redisContext *c, const char *format, ...);
-void *redisCommandWithCallback(redisContext *c, redisCallbackFn *fn, void *privdata, const char *format, ...);
+void *redisCommandWithCallback(redisContext *c, redisCallbackFn *fn, const void *privdata, const char *format, ...);
 
 #endif