diff --git a/editors/sed.c b/editors/sed.c
index 1054c13..cddb0c7 100644
--- a/editors/sed.c
+++ b/editors/sed.c
@@ -387,7 +387,7 @@
 		bb_error_msg_and_die("empty filename");
 	*retval = xstrndup(filecmdstr+start, idx-start+hack+1);
 	if (hack)
-		(*retval)[idx] = '\\';
+		(*retval)[idx-start] = '\\';
 
 	return idx;
 }