diff --git a/src/mscorlib/src/System/String.cs b/src/mscorlib/src/System/String.cs
index 49c3f317e4d5..8acafd6d5fc4 100644
--- a/src/mscorlib/src/System/String.cs
+++ b/src/mscorlib/src/System/String.cs
@@ -159,27 +159,27 @@ public static String Join(String separator, IEnumerable<String> values) {
             Contract.Ensures(Contract.Result<String>() != null);
             Contract.EndContractBlock();
 
-            if (separator == null)
-                separator = String.Empty;
-
-
             using(IEnumerator<String> en = values.GetEnumerator()) {
                 if (!en.MoveNext())
                     return String.Empty;
 
-                StringBuilder result = StringBuilderCache.Acquire();
-                if (en.Current != null) {
-                    result.Append(en.Current);
+                String firstValue = en.Current;
+
+                if (!en.MoveNext()) {
+                    // Only one value available
+                    return firstValue ?? String.Empty;
                 }
 
-                while (en.MoveNext()) {
+                // Null separator and values are handled by the StringBuilder
+                StringBuilder result = StringBuilderCache.Acquire();
+                result.Append(firstValue);
+
+                do {
                     result.Append(separator);
-                    if (en.Current != null) {
-                        result.Append(en.Current);
-                    }
-                }            
+                    result.Append(en.Current);
+                } while (en.MoveNext());
                 return StringBuilderCache.GetStringAndRelease(result);
-            }           
+            }
         }
 
 
@@ -219,13 +219,19 @@ public unsafe static String Join(String separator, String[] value, int startInde
             if (count == 0) {
                 return String.Empty;
             }
-            
+
+            if (count == 1) {
+                return value[startIndex] ?? String.Empty;
+            }
+
             int jointLength = 0;
             //Figure out the total length of the strings in value
             int endIndex = startIndex + count - 1;
             for (int stringToJoinIndex = startIndex; stringToJoinIndex <= endIndex; stringToJoinIndex++) {
-                if (value[stringToJoinIndex] != null) {
-                    jointLength += value[stringToJoinIndex].Length;
+                string currentValue = value[stringToJoinIndex];
+
+                if (currentValue != null) {
+                    jointLength += currentValue.Length;
                 }
             }