diff --git a/src/main/java/java/lang/String.java b/src/main/java/java/lang/String.java
index 0bd0854..8f4527e 100644
--- a/src/main/java/java/lang/String.java
+++ b/src/main/java/java/lang/String.java
@@ -3873,9 +3873,7 @@ public static String copyValueOf(char data[]) {
      * @diffblue.untested
      */
     public static String valueOf(boolean b) {
-        // DIFFBLUE MODEL LIBRARY This is treated internally in CBMC
-        return CProver.nondetWithoutNullForNotModelled();
-        // return b ? "true" : "false";
+        return b ? "true" : "false";
     }
 
     /**
diff --git a/src/main/java/java/lang/StringBuffer.java b/src/main/java/java/lang/StringBuffer.java
index 7d3c120..b89d4e4 100644
--- a/src/main/java/java/lang/StringBuffer.java
+++ b/src/main/java/java/lang/StringBuffer.java
@@ -505,11 +505,11 @@ public synchronized StringBuffer append(char[] str, int offset, int len) {
      */
     @Override
     public synchronized StringBuffer append(boolean b) {
-        // DIFFBLUE MODEL LIBRARY this is replaced internally
+        // DIFFBLUE MODEL LIBRARY do not use cache
         // toStringCache = null;
         // super.append(b);
         // return this;
-        return CProver.nondetWithoutNullForNotModelled();
+        return append(b ? "true" : "false");
     }
 
     /**