diff --git a/ChangeLog b/ChangeLog
index 2bb0a2b4f8..7c545d08a3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Sat Sep 15 17:04:08 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* encoding.c (rb_enc_associate_index, rb_enc_get_index): check if
+	  object is encoding capable.  [ruby-dev:31780]
+
+	* string.c (rb_str_subpat_set): check for if the argument is a String.
+
 Sat Sep 15 13:31:21 2007  Kouhei Sutou  <kou@cozmixng.org>
 
 	* lib/rss.rb, lib/rss/, test/rss/:
diff --git a/encoding.c b/encoding.c
index 8b674d13ea..b1c8ba9349 100644
--- a/encoding.c
+++ b/encoding.c
@@ -79,9 +79,49 @@ rb_enc_find(const char *name)
     return ONIG_ENCODING_ASCII;
 }
 
+static int
+enc_capable(VALUE obj)
+{
+    if (IMMEDIATE_P(obj)) return Qfalse;
+    switch (BUILTIN_TYPE(obj)) {
+      case T_STRING:
+      case T_REGEXP:
+      case T_FILE:
+	return Qtrue;
+      default:
+	return Qfalse;
+    }
+}
+
+static void
+enc_check_capable(VALUE x)
+{
+    if (!enc_capable(x)) {
+	const char *etype;
+
+	if (NIL_P(x)) {
+	    etype = "nil";
+	}
+	else if (FIXNUM_P(x)) {
+	    etype = "Fixnum";
+	}
+	else if (SYMBOL_P(x)) {
+	    etype = "Symbol";
+	}
+	else if (rb_special_const_p(x)) {
+	    etype = RSTRING_PTR(rb_obj_as_string(x));
+	}
+	else {
+	    etype = rb_obj_classname(x);
+	}
+	rb_raise(rb_eTypeError, "wrong argument type %s (not encode capable)", etype);
+    }
+}
+
 void
 rb_enc_associate_index(VALUE obj, int idx)
 {
+    enc_check_capable(obj);
     if (idx < ENCODING_INLINE_MAX) {
 	ENCODING_SET(obj, idx);
 	return;
@@ -117,8 +157,10 @@ rb_enc_associate(VALUE obj, rb_encoding *enc)
 int
 rb_enc_get_index(VALUE obj)
 {
-    int i = ENCODING_GET(obj);
+    int i;
 
+    enc_check_capable(obj);
+    i = ENCODING_GET(obj);
     if (i == ENCODING_INLINE_MAX) {
 	VALUE iv;
 
diff --git a/string.c b/string.c
index 3eaeb9e5db..e20007f0b9 100644
--- a/string.c
+++ b/string.c
@@ -1996,6 +1996,7 @@ rb_str_subpat_set(VALUE str, VALUE re, int nth, VALUE val)
     }
     end = RMATCH(match)->END(nth);
     len = end - start;
+    StringValue(val);
     rb_enc_check(str, val);
     rb_str_splice_0(str, start, len, val);
 }