diff --git a/ext/java/org/jruby/ext/stringio/StringIO.java b/ext/java/org/jruby/ext/stringio/StringIO.java
index ed53fdd..acbb5db 100644
--- a/ext/java/org/jruby/ext/stringio/StringIO.java
+++ b/ext/java/org/jruby/ext/stringio/StringIO.java
@@ -1110,11 +1110,7 @@ private IRubyObject readCommon(ThreadContext context, int argc, IRubyObject arg0
             final RubyString string;
             switch (argc) {
                 case 2:
-                    str = arg1;
-                    if (!str.isNil()) {
-                        str = str.convertToString();
-                        modifyString((RubyString) str);
-                    }
+                    str = readBuf(arg1);
                 case 1:
                     if (!arg0.isNil()) {
                         len = RubyNumeric.fix2int(arg0);
@@ -1206,11 +1202,7 @@ private RubyString preadCommon(ThreadContext context, int argc, IRubyObject arg0
 
         switch (argc) {
             case 3:
-                str = arg2;
-                if (!str.isNil()) {
-                    str = str.convertToString();
-                    modifyString((RubyString) str);
-                }
+                str = readBuf(arg2);
             case 2:
                 len = RubyNumeric.fix2int(arg0);
                 if (!arg0.isNil()) {
@@ -1702,6 +1694,14 @@ private static IRubyObject substrString(RubyString ch, IRubyObject str, Ruby run
         return str;
     }
 
+    private static IRubyObject readBuf(IRubyObject str) {
+        if (!str.isNil()) {
+            str = str.convertToString();
+            modifyString((RubyString) str);
+        }
+        return str;
+    }
+
     // MRI: strio_write
     private long stringIOWrite(ThreadContext context, Ruby runtime, IRubyObject arg) {
         checkWritable();