diff --git a/src/tool_cb_dbg.c b/src/tool_cb_dbg.c
index f4252ad..4eeb7b2 100644
--- a/src/tool_cb_dbg.c
+++ b/src/tool_cb_dbg.c
@@ -41,7 +41,7 @@ static void dump(const char *timebuf, const char *text,
 */
 
 int tool_debug_cb(CURL *handle, curl_infotype type,
-                  unsigned char *data, size_t size,
+                  char *data, size_t size,
                   void *userdata)
 {
   struct OperationConfig *operation = userdata;
@@ -209,7 +209,8 @@ int tool_debug_cb(CURL *handle, curl_infotype type,
     break;
   }
 
-  dump(timebuf, text, output, data, size, config->tracetype, type);
+  dump(timebuf, text, output, (unsigned char *) data, size, config->tracetype,
+       type);
   return 0;
 }
 
diff --git a/src/tool_cb_dbg.h b/src/tool_cb_dbg.h
index 7ac15cf..c1cbc80 100644
--- a/src/tool_cb_dbg.h
+++ b/src/tool_cb_dbg.h
@@ -28,7 +28,7 @@
 */
 
 int tool_debug_cb(CURL *handle, curl_infotype type,
-                  unsigned char *data, size_t size,
+                  char *data, size_t size,
                   void *userdata);
 
 #endif /* HEADER_CURL_TOOL_CB_DBG_H */