Index: ext/stringio/stringio.c
===================================================================
--- ext/stringio/stringio.c	(revision 45637)
+++ ext/stringio/stringio.c	(working copy)
@@ -1191,8 +1191,8 @@
     else {
 	strio_extend(ptr, ptr->pos, len);
 	memmove(RSTRING_PTR(ptr->string)+ptr->pos, RSTRING_PTR(str), len);
-	OBJ_INFECT(ptr->string, str);
     }
+    OBJ_INFECT(ptr->string, str);
     OBJ_INFECT(ptr->string, self);
     ptr->pos += len;
     return LONG2NUM(len);