diff --git a/ext/interbase/ibase_service.c b/ext/interbase/ibase_service.c
index c50f370e623..474b7a490fb 100644
--- a/ext/interbase/ibase_service.c
+++ b/ext/interbase/ibase_service.c
@@ -322,7 +322,7 @@ query_loop:
 					heap_p = heap_buf + res_size;
 				}
 				result += 2;
-				snprintf(heap_p, sizeof(heap_buf_size), "%s\n", result);
+				snprintf(heap_p, heap_buf_size - (heap_buf - heap_p), "%s\n", result);
 				heap_p += line_len +2;
 				goto query_loop; /* repeat until result is exhausted */