diff --git a/moshi/src/main/java/com/squareup/moshi/Moshi.java b/moshi/src/main/java/com/squareup/moshi/Moshi.java
index 64844ed..39e83f3 100644
--- a/moshi/src/main/java/com/squareup/moshi/Moshi.java
+++ b/moshi/src/main/java/com/squareup/moshi/Moshi.java
@@ -182,11 +182,13 @@ public final class Moshi {
     }
 
     public Builder add(JsonAdapter.Factory factory) {
+      if (factory == null) throw new IllegalArgumentException("factory == null");
       factories.add(factory);
       return this;
     }
 
     public Builder add(Object adapter) {
+      if (adapter == null) throw new IllegalArgumentException("adapter == null");
       return add(AdapterMethodsFactory.get(adapter));
     }
 
diff --git a/moshi/src/test/java/com/squareup/moshi/MoshiTest.java b/moshi/src/test/java/com/squareup/moshi/MoshiTest.java
index ab8b6f5..cb0169c 100644
--- a/moshi/src/test/java/com/squareup/moshi/MoshiTest.java
+++ b/moshi/src/test/java/com/squareup/moshi/MoshiTest.java
@@ -532,6 +532,63 @@ public final class MoshiTest {
     assertThat(adapter.toJson(null)).isEqualTo("null");
   }
 
+  @Test public void addNullFails() throws Exception {
+    JsonAdapter jsonAdapter = new JsonAdapter() {
+      @Override public Object fromJson(JsonReader reader) throws IOException {
+        throw new AssertionError();
+      }
+
+      @Override public void toJson(JsonWriter writer, Object value) throws IOException {
+        throw new AssertionError();
+      }
+    };
+    Type type = Object.class;
+    Class<? extends Annotation> annotation = Annotation.class;
+    Moshi.Builder builder = new Moshi.Builder();
+    try {
+      builder.add((null));
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("factory == null");
+    }
+    try {
+      builder.add((Object) null);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("adapter == null");
+    }
+    try {
+      builder.add(null, null);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("type == null");
+    }
+    try {
+      builder.add(type, null);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("jsonAdapter == null");
+    }
+    try {
+      builder.add(null, null, null);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("type == null");
+    }
+    try {
+      builder.add(type, null, null);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("annotation == null");
+    }
+    try {
+      builder.add(type, annotation, null);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("jsonAdapter == null");
+    }
+  }
+
   @Test public void customJsonAdapter() throws Exception {
     Moshi moshi = new Moshi.Builder()
         .add(Pizza.class, new PizzaAdapter())