diff --git a/types/knockout/index.d.ts b/types/knockout/index.d.ts
index 1340e22d8f..fd30672d93 100644
--- a/types/knockout/index.d.ts
+++ b/types/knockout/index.d.ts
@@ -106,12 +106,14 @@ interface KnockoutObservableArray<T> extends KnockoutObservable<T[]>, KnockoutOb
 interface KnockoutObservableStatic {
     fn: KnockoutObservableFunctions<any>;
 
-    <T>(value?: T | null): KnockoutObservable<T>;
+    <T = any>(): KnockoutObservable<T | undefined>
+    <T = any>(value: null): KnockoutObservable<T | null>
+    <T>(value: T): KnockoutObservable<T>;
 }
 
 interface KnockoutObservable<T> extends KnockoutSubscribable<T>, KnockoutObservableFunctions<T> {
     (): T;
-    (value: T | null): void;
+    (value: T): void;
 
     peek(): T;
     valueHasMutated?:{(): void;};
diff --git a/types/knockout/test/templatingBehaviors.ts b/types/knockout/test/templatingBehaviors.ts
index b69cb080f8..bf87b784da 100644
--- a/types/knockout/test/templatingBehaviors.ts
+++ b/types/knockout/test/templatingBehaviors.ts
@@ -575,7 +575,8 @@ describe('Templating', function() {
 
         // Now set the observable to null and check it's treated like an empty array
         // (because how else should null be interpreted?)
-        myArray(null);
+        // DefinitelyTyped note: while KO accepts this, I wouldn't consider it a well-typed usage of the API
+        myArray(null); // $ExpectError
         expect(testNode.childNodes[0].childNodes.length).toEqual(0);
     });
 
@@ -640,7 +641,7 @@ describe('Templating', function() {
         ko.setTemplateEngine(new dummyTemplateEngine({ myTemplate: "Value: [js: myProp().childProp]" }));
         testNode.innerHTML = "<div data-bind='template: { name: \"myTemplate\", \"if\": myProp }'></div>";
 
-        var viewModel = { myProp: ko.observable({ childProp: 'abc' }) };
+        var viewModel = { myProp: ko.observable<{childProp: string} | null>({ childProp: 'abc' }) };
         ko.applyBindings(viewModel, testNode);
 
         // Initially there is a value
@@ -678,7 +679,7 @@ describe('Templating', function() {
         ko.setTemplateEngine(new dummyTemplateEngine({ myTemplate: "Value: [js: myProp().childProp]" }));
         testNode.innerHTML = "<div data-bind='template: { name: \"myTemplate\", \"if\": myProp, foreach: [$data, $data, $data] }'></div>";
 
-        var viewModel = { myProp: ko.observable({ childProp: 'abc' }) };
+        var viewModel = { myProp: ko.observable<{childProp: string} | null>({ childProp: 'abc' }) };
         ko.applyBindings(viewModel, testNode);
         expect(testNode.childNodes[0].childNodes[0].nodeValue).toEqual("Value: abc");
         expect(testNode.childNodes[0].childNodes[1].nodeValue).toEqual("Value: abc");
diff --git a/types/valerie/valerie-tests.ts b/types/valerie/valerie-tests.ts
index b4f6efcdc9..2121b20c64 100644
--- a/types/valerie/valerie-tests.ts
+++ b/types/valerie/valerie-tests.ts
@@ -9,7 +9,7 @@ enum enumTest { male, female }
  */
 function ObservableValidationTypes() {
     // any
-    var t0 = ko.observable<any>()
+    var t0 = ko.observable()
         .validate()
         .end();