diff --git a/cmPrefs.c b/cmPrefs.c
index 486c8a6..524adfd 100644
--- a/cmPrefs.c
+++ b/cmPrefs.c
@@ -305,6 +305,8 @@ cmPrRC_t cmPrefsInitialize( cmPrH_t* hp, const cmChar_t* fn, cmPrefsOnChangeFunc
   p->fn = cmLHeapAllocZ( p->lhH, strlen(fn)+1 );
   strcpy(p->fn,fn);
 
+  p->id = kMaxVarPrId;
+
   hp->h = p;
 
  errLabel:
@@ -1169,6 +1171,10 @@ cmPrRC_t  _cmPrefsCreateJsonNode(
   int             i;
 
   assert( pathCnt >= 1 );
+
+  if( id != cmInvalidId && id >= kMaxVarPrId )
+    return cmErrMsg(&p->err,kNodeCreateFailPrRC,"User supplied id's must be less than 0x%x.",kMaxVarPrId);
+
   
   // for each path element
   for(i=0; i<pathCnt; ++i)
diff --git a/cmPrefs.h b/cmPrefs.h
index 41def74..7b8dcd1 100644
--- a/cmPrefs.h
+++ b/cmPrefs.h
@@ -24,6 +24,12 @@ extern "C" {
     kDuplicateIdPrRC
   };
 
+  enum
+  {
+    kMaxVarPrId = 0x7fffffff // User assigned variable id's given to cmPrefsCreateXXX()
+                             // must be less than kMaxVarPrId
+  };
+
   typedef void (*cmPrefsOnChangeFunc_t)( cmPrH_t h, void* cbDataPtr, unsigned id ); 
 
   extern cmPrH_t cmPrNullHandle;
@@ -139,7 +145,8 @@ extern "C" {
   // The 'id' argument is optional.  If 'id' is set to cmInvalidId then the
   // variable will be automatically assigned an id.  The value of the
   // automatically assigned id can be found from the path string
-  // via cmPrefsId()
+  // via cmPrefsId().  If 'id' is not set to cmInvalidId then it must be less than 
+  // kMaxVarId.
 
   // Set kForceValuePrFl 
   enum { kForceValuePrFl=0x01 };