Don't break in shrink and expand when the generator produces falsy va…

…lues

src/PickoneGenerator.js

@@ -15,7 +15,11 @@ class PickoneGenerator extends Generator {
   }
 
   shrink(item) {
-    if (this.lastUnwrappedValue === item && this.lastValue.shrink) {
+    if (
+      this.lastUnwrappedValue === item &&
+      this.lastValue &&
+      this.lastValue.shrink
+    ) {
       return this.lastValue.shrink(item);
     } else {
       return new ConstantGenerator(item);
@@ -24,7 +28,9 @@ class PickoneGenerator extends Generator {
 
   expand(item) {
     const expandableItem =
-      this.lastUnwrappedValue === item && this.lastValue.expand;
+      this.lastUnwrappedValue === item &&
+      this.lastValue &&
+      this.lastValue.expand;
 
     return new WeightedGenerator([
       [this, 10],

src/PickoneGenerator.spec.js

@@ -38,12 +38,20 @@ describe("PickoneGenerator", () => {
         ConstantGenerator
       );
     });
+
+    it("does not break when the generator emits falsy values", () => {
+      new PickoneGenerator([null]).shrink().first();
+    });
   });
 
   describe("expand", () => {
     it("returns a new generator that is more likely to pick the given item", () => {
       expect(generator.expand(5), "to yield items", [7, 5, 7, 5, 5, 5, 4, 0]);
     });
+
+    it("does not break when the generator emits falsy values", () => {
+      new PickoneGenerator([null]).expand().first();
+    });
   });
 
   describe("when the items contains generators", () => {