diff --git a/ext/java/org/jruby/ext/stringio/StringIO.java b/ext/java/org/jruby/ext/stringio/StringIO.java
index c76fd25e..13329a02 100644
--- a/ext/java/org/jruby/ext/stringio/StringIO.java
+++ b/ext/java/org/jruby/ext/stringio/StringIO.java
@@ -149,7 +149,7 @@ public Encoding getEncoding() {
         }
 
         RubyString string = ptr.string;
-        if (string != null && !string.isNil()) {
+        if (string != null) {
             return string.getEncoding();
         }
 
@@ -1067,19 +1067,19 @@ public IRubyObject putc(ThreadContext context, IRubyObject ch) {
 
     public static final ByteList NEWLINE = ByteList.create("\n");
 
-    @JRubyMethod(name = "read")
+    @JRubyMethod(name = "read") // strio.read()
     public IRubyObject read(ThreadContext context) {
         return readCommon(context, 0, null, null);
     }
 
-    @JRubyMethod(name = "read")
-    public IRubyObject read(ThreadContext context, IRubyObject arg0) {
-        return readCommon(context, 1, arg0, null);
+    @JRubyMethod(name = "read") // strio.read(length)
+    public IRubyObject read(ThreadContext context, IRubyObject length) {
+        return readCommon(context, 1, length, null);
     }
 
-    @JRubyMethod(name = "read")
-    public IRubyObject read(ThreadContext context, IRubyObject arg0, IRubyObject arg1) {
-        return readCommon(context, 2, arg0, arg1);
+    @JRubyMethod(name = "read") // strio.read(length, outbuf)
+    public IRubyObject read(ThreadContext context, IRubyObject length, IRubyObject outbuf) {
+        return readCommon(context, 2, length, outbuf);
     }
 
     @SuppressWarnings("fallthrough")
@@ -1102,7 +1102,7 @@ private IRubyObject readCommon(ThreadContext context, int argc, IRubyObject arg0
                     str = arg1;
                     if (!str.isNil()) {
                         str = str.convertToString();
-                        ((RubyString) str).modify();
+                        modifyString((RubyString) str);
                     }
                 case 1:
                     if (!arg0.isNil()) {
@@ -1198,7 +1198,7 @@ private RubyString preadCommon(ThreadContext context, int argc, IRubyObject arg0
                 str = arg2;
                 if (!str.isNil()) {
                     str = str.convertToString();
-                    ((RubyString) str).modify();
+                    modifyString((RubyString) str);
                 }
             case 2:
                 len = RubyNumeric.fix2int(arg0);
@@ -2210,12 +2210,13 @@ private void checkWritable() {
     }
 
     private void checkModifiable() {
-        if (getPtr().string == null || getPtr().string.isNil()) {
+        final RubyString string = getPtr().string;
+        if (string == null) {
             /* Null device StringIO */
-        } else if (getPtr().string.isFrozen()) {
+        } else if (string.isFrozen()) {
             throw getRuntime().newIOError("not modifiable string");
         } else {
-            getPtr().string.modify();
+            string.modify();
         }
     }
 
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 70bab8af..fe890406 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -1064,6 +1064,20 @@ def test_coderange_after_overwrite
     assert_predicate(s.string, :ascii_only?)
   end
 
+  def test_coderange_after_read_into_buffer
+    s = StringIO.new("01234567890".b)
+
+    buf = "¿Cómo estás? Ça va bien?"
+    assert_not_predicate(buf, :ascii_only?)
+
+    assert_predicate(s.string, :ascii_only?)
+
+    s.read(10, buf)
+
+    assert_predicate(buf, :ascii_only?)
+    assert_equal '0123456789', buf
+  end
+
   require "objspace"
   if ObjectSpace.respond_to?(:dump) && ObjectSpace.dump(eval(%{"test"})).include?('"chilled":true') # Ruby 3.4+ chilled strings
     def test_chilled_string