diff --git a/spec/tags/1.9/ruby/core/string/concat_tags.txt b/spec/tags/1.9/ruby/core/string/concat_tags.txt
index a6dbfa04444..8baa2501382 100644
--- a/spec/tags/1.9/ruby/core/string/concat_tags.txt
+++ b/spec/tags/1.9/ruby/core/string/concat_tags.txt
@@ -1,3 +1 @@
-fails:String#concat with Integer returns a ASCII-8BIT string if self is US-ASCII and the argument is between 128-255 (inclusive)
-fails:String#concat with Integer raises RangeError if the argument is an invalid codepoint for self's encoding
 fails:String#concat when self is ASCII-8BIT and argument is US-ASCII uses ASCII-8BIT encoding
diff --git a/src/org/jruby/RubyString.java b/src/org/jruby/RubyString.java
index fc1eda9bb6a..1e716a4a531 100644
--- a/src/org/jruby/RubyString.java
+++ b/src/org/jruby/RubyString.java
@@ -69,6 +69,7 @@
 import org.jcodings.EncodingDB;
 import org.jcodings.ascii.AsciiTables;
 import org.jcodings.constants.CharacterType;
+import org.jcodings.exception.EncodingException;
 import org.jcodings.specific.ASCIIEncoding;
 import org.jcodings.specific.USASCIIEncoding;
 import org.jcodings.specific.UTF8Encoding;
@@ -2638,9 +2639,23 @@ public RubyString concat19(ThreadContext context, IRubyObject other) {
 
     private RubyString concatNumeric(Ruby runtime, int c) {
         Encoding enc = value.getEncoding();
-        int cl = codeLength(runtime, enc, c);
-        modify19(value.getRealSize() + cl);
-        enc.codeToMbc(c, value.getUnsafeBytes(), value.getBegin() + value.getRealSize());
+        int cl;
+
+        try {
+            cl = codeLength(runtime, enc, c);
+            modify19(value.getRealSize() + cl);
+
+            if (enc == USASCIIEncoding.INSTANCE) {
+                if (c > 0xff) runtime.newRangeError(c + " out of char range");
+                if (c > 0x79) {
+                    value.setEncoding(ASCIIEncoding.INSTANCE);
+                    enc = value.getEncoding();
+                }
+            }
+            enc.codeToMbc(c, value.getUnsafeBytes(), value.getBegin() + value.getRealSize());
+        } catch (EncodingException e) {
+            throw runtime.newRangeError(c + " out of char range");
+        }
         value.setRealSize(value.getRealSize() + cl);
         return this;
     }