diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index c2b600c..05d4721 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -540,10 +540,11 @@
 	if (!unique || i == 0
 		|| (sort_ic ? STRICMP(s, sortbuf1) : STRCMP(s, sortbuf1)) != 0)
 	{
-	    if (ml_append(lnum++, s, (colnr_T)0, FALSE) == FAIL)
+	    /* Copy the line into a buffer, it may become invalid in
+	     * ml_append(). And it's needed for "unique". */
+	    STRCPY(sortbuf1, s);
+	    if (ml_append(lnum++, sortbuf1, (colnr_T)0, FALSE) == FAIL)
 		break;
-	    if (unique)
-		STRCPY(sortbuf1, s);
 	}
 	fast_breakcheck();
 	if (got_int)