diff --git a/src/backend.c b/src/backend.c
index f49e388..f88240d 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -89,24 +89,25 @@ f2b_backend_destroy(f2b_backend_t *backend) {
   free(backend);
 }
 
-#define BACKEND_CMD_ARG0(CMD) \
-bool \
+#define BACKEND_CMD_ARG0(CMD, RETURNS) \
+RETURNS \
 f2b_backend_ ## CMD(f2b_backend_t *backend) { \
   assert(backend != NULL); \
   return backend->CMD(backend->cfg); \
 }
 
-#define BACKEND_CMD_ARG1(CMD) \
-bool \
+#define BACKEND_CMD_ARG1(CMD, RETURNS) \
+RETURNS \
 f2b_backend_ ## CMD(f2b_backend_t *backend, const char *ip) { \
   assert(backend != NULL); \
   return backend->CMD(backend->cfg, ip); \
 }
 
-BACKEND_CMD_ARG0(ping)
-BACKEND_CMD_ARG0(start)
-BACKEND_CMD_ARG0(stop)
+BACKEND_CMD_ARG0(error, const char *)
+BACKEND_CMD_ARG0(start, bool)
+BACKEND_CMD_ARG0(stop,  bool)
+BACKEND_CMD_ARG0(ping,  bool)
 
-BACKEND_CMD_ARG1(check)
-BACKEND_CMD_ARG1(ban)
-BACKEND_CMD_ARG1(unban)
+BACKEND_CMD_ARG1(check, bool)
+BACKEND_CMD_ARG1(ban,   bool)
+BACKEND_CMD_ARG1(unban, bool)
diff --git a/src/backend.h b/src/backend.h
index d5b2e51..90723d4 100644
--- a/src/backend.h
+++ b/src/backend.h
@@ -10,7 +10,7 @@ typedef struct f2b_backend_t {
   void *(*create)  (const char *id);
   bool  (*config)  (void *cfg, const char *key, const char *value);
   bool  (*ready)   (void *cfg);
-  char  (*error)   (void *cfg);
+  char *(*error)   (void *cfg);
   bool  (*start)   (void *cfg);
   bool  (*stop)    (void *cfg);
   bool  (*ping)    (void *cfg);