diff --git a/src/autocmd.c b/src/autocmd.c
index aa18087..f0b573b 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -870,7 +870,7 @@
     if (*what == ',' && *p_ei == NUL)
 	STRCPY(new_ei, what + 1);
     else
-	STRCAT(new_ei, what);
+	STRCPY(new_ei + p_ei_len, what);
     set_string_option_direct((char_u *)"ei", -1, new_ei,
 	    OPT_FREE, SID_NONE);
     vim_free(new_ei);