diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 4f1d932..566ed7d 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -4519,6 +4519,9 @@
 		{
 		    nmatch = curbuf->b_ml.ml_line_count - sub_firstlnum + 1;
 		    skip_match = TRUE;
+		    // safety check
+		    if (nmatch < 0)
+			goto skip;
 		}
 
 		// Need room for: