diff --git a/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/Key.java b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/Key.java
index 183092f48b42..bbe07f54f957 100644
--- a/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/Key.java
+++ b/google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/Key.java
@@ -281,7 +281,9 @@ ListValue toProto() {
       } else if (part instanceof ByteArray) {
         builder.addValuesBuilder().setStringValue(((ByteArray) part).toBase64());
       } else if (part instanceof Timestamp) {
-        builder.addValuesBuilder().setStringValue(((Timestamp) part).toString());
+        builder.addValuesBuilder().setStringValue(part.toString());
+      } else if (part instanceof Date) {
+        builder.addValuesBuilder().setStringValue(part.toString());
       } else {
         throw new AssertionError("Illegal key part: " + part.getClass());
       }
diff --git a/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/KeyTest.java b/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/KeyTest.java
index 78012912fefa..81f3957c286a 100644
--- a/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/KeyTest.java
+++ b/google-cloud-clients/google-cloud-spanner/src/test/java/com/google/cloud/spanner/KeyTest.java
@@ -23,6 +23,8 @@
 import com.google.cloud.Date;
 import com.google.cloud.Timestamp;
 import com.google.common.testing.EqualsTester;
+import com.google.protobuf.ListValue;
+import com.google.protobuf.NullValue;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -189,4 +191,35 @@ public void serialization() throws Exception {
     reserializeAndAssert(Key.of(Date.parseDate("2015-09-15")));
     reserializeAndAssert(Key.of(1, 2, 3));
   }
+
+  @Test
+  public void toProto() {
+    String timestamp = "2015-09-15T00:00:00Z";
+    String date = "2015-09-15";
+    Key k =
+        Key.newBuilder()
+            .append((Boolean) null)
+            .append(true)
+            .append(32)
+            .append(64L)
+            .append(2.0f)
+            .append(4.0d)
+            .append("x")
+            .append(ByteArray.copyFrom("y"))
+            .append(Timestamp.parseTimestamp(timestamp))
+            .append(Date.parseDate(date))
+            .build();
+    ListValue.Builder builder = ListValue.newBuilder();
+    builder.addValuesBuilder().setNullValue(NullValue.NULL_VALUE);
+    builder.addValuesBuilder().setBoolValue(true);
+    builder.addValuesBuilder().setStringValue("32");
+    builder.addValuesBuilder().setStringValue("64");
+    builder.addValuesBuilder().setNumberValue(2.0f);
+    builder.addValuesBuilder().setNumberValue(4.0d);
+    builder.addValuesBuilder().setStringValue("x");
+    builder.addValuesBuilder().setStringValue("eQ==");
+    builder.addValuesBuilder().setStringValue(timestamp);
+    builder.addValuesBuilder().setStringValue(date);
+    assertThat(k.toProto()).isEqualTo(builder.build());
+  }
 }