diff --git a/src/ops.c b/src/ops.c
index 1ddff28..146c990 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -2602,7 +2602,8 @@
 	firstline = ml_get(oap->start.lnum) + bd.textcol;
 	if (oap->op_type == OP_APPEND)
 	    firstline += bd.textlen;
-	if ((ins_len = (long)STRLEN(firstline) - pre_textlen) > 0)
+	if (pre_textlen >= 0
+		     && (ins_len = (long)STRLEN(firstline) - pre_textlen) > 0)
 	{
 	    ins_text = vim_strnsave(firstline, (int)ins_len);
 	    if (ins_text != NULL)