diff --git a/contrib/prometheus-exporter/service-prometheus.c b/contrib/prometheus-exporter/service-prometheus.c
index b57af99..2aafd4d 100644
--- a/contrib/prometheus-exporter/service-prometheus.c
+++ b/contrib/prometheus-exporter/service-prometheus.c
@@ -1408,9 +1408,11 @@
 	}
 
   end:
-	if (!htx_add_data_atonce(htx, out))
-		return -1; /* Unexpected and unrecoverable error */
-	channel_add_input(chn, out.len);
+	if (out.len) {
+		if (!htx_add_data_atonce(htx, out))
+			return -1; /* Unexpected and unrecoverable error */
+		channel_add_input(chn, out.len);
+	}
 	return ret;
   full:
 	ret = 0;
@@ -1586,9 +1588,11 @@
 	}
 
   end:
-	if (!htx_add_data_atonce(htx, out))
-		return -1; /* Unexpected and unrecoverable error */
-	channel_add_input(chn, out.len);
+	if (out.len) {
+		if (!htx_add_data_atonce(htx, out))
+			return -1; /* Unexpected and unrecoverable error */
+		channel_add_input(chn, out.len);
+	}
 	return ret;
   full:
 	ret = 0;
@@ -1804,9 +1808,11 @@
 	}
 
   end:
-	if (!htx_add_data_atonce(htx, out))
-		return -1; /* Unexpected and unrecoverable error */
-	channel_add_input(chn, out.len);
+	if (out.len) {
+		if (!htx_add_data_atonce(htx, out))
+			return -1; /* Unexpected and unrecoverable error */
+		channel_add_input(chn, out.len);
+	}
 	return ret;
   full:
 	ret = 0;
@@ -1998,9 +2004,11 @@
 
 
   end:
-	if (!htx_add_data_atonce(htx, out))
-		return -1; /* Unexpected and unrecoverable error */
-	channel_add_input(chn, out.len);
+	if (out.len) {
+		if (!htx_add_data_atonce(htx, out))
+			return -1; /* Unexpected and unrecoverable error */
+		channel_add_input(chn, out.len);
+	}
 	return ret;
   full:
 	ret = 0;