diff --git a/observers.js b/observers.js
index 09434b2..2d08534 100644
--- a/observers.js
+++ b/observers.js
@@ -326,7 +326,7 @@ exports.makeDefinedObserver = makeDefinedObserver;
 function makeDefinedObserver(observeValue) {
     return function observeDefault(emit, scope) {
         return observeValue(autoCancelPrevious(function replaceValue(value) {
-            return emit(value != null);
+            return emit(Operators.defined(value));
         }), scope);
     };
 }
diff --git a/operators.js b/operators.js
index 058dfb0..c836872 100644
--- a/operators.js
+++ b/operators.js
@@ -111,7 +111,9 @@ exports.or = function (a, b) {
 };
 
 exports.defined = function (value) {
-    return value != null;
+    // value != null eliminates null and undefined
+    // value === value eliminates NaN
+    return value != null && value === value;
 };
 
 // "startsWith", "endsWith", and "contains"  are overridden in