diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index c30b6fd..53c7bb5 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -4651,6 +4651,9 @@
 		mch_memmove(new_end, sub_firstline + copycol, (size_t)copy_len);
 		new_end += copy_len;
 
+		if (new_start_len - copy_len < sublen)
+		    sublen = new_start_len - copy_len - 1;
+
 #ifdef FEAT_EVAL
 		++textlock;
 #endif