diff --git a/libs/freetdm/src/zap_buffer.c b/libs/freetdm/src/zap_buffer.c
index 9cfb354ad4..525a3e2100 100644
--- a/libs/freetdm/src/zap_buffer.c
+++ b/libs/freetdm/src/zap_buffer.c
@@ -187,7 +187,7 @@ zap_size_t zap_buffer_write(zap_buffer_t *buffer, const void *data, zap_size_t d
 	}
 
 	actual_freespace = buffer->datalen - buffer->actually_used;
-	if (actual_freespace < datalen && (!buffer->max_len || (buffer->datalen + datalen <= buffer->max_len))) {
+	if (actual_freespace < datalen && (!buffer->max_len || (buffer->used + datalen <= buffer->max_len))) {
 		memmove(buffer->data, buffer->head, buffer->used);
 		buffer->head = buffer->data;
 		buffer->actually_used = buffer->used;