diff --git a/lib/src/firestore-to-proto.spec.ts b/lib/src/firestore-to-proto.spec.ts
index 45d392009..f3182d465 100644
--- a/lib/src/firestore-to-proto.spec.ts
+++ b/lib/src/firestore-to-proto.spec.ts
@@ -110,6 +110,34 @@ describe('toMessage()', () => {
         },
       }),
     },
+    {
+      desc: 'skips null nested message',
+      input: {
+        '4': null,
+      },
+      expected: new Job(),
+    },
+    {
+      desc: 'skips null map field',
+      input: {
+        '4': null,
+      },
+      expected: new Survey(),
+    },
+    {
+      desc: 'skips non-object map field',
+      input: {
+        '4': 'not an object',
+      },
+      expected: new Survey(),
+    },
+    {
+      desc: 'skips array map field',
+      input: {
+        '4': [1, 2, 3],
+      },
+      expected: new Survey(),
+    },
     {
       desc: 'converts enum value',
       input: {
diff --git a/lib/src/firestore-to-proto.ts b/lib/src/firestore-to-proto.ts
index cc8309e14..52fc9a4c4 100644
--- a/lib/src/firestore-to-proto.ts
+++ b/lib/src/firestore-to-proto.ts
@@ -77,10 +77,11 @@ function toMessageValue(
   if (!descriptor.fields) return null;
   const field = descriptor.fields[fieldName];
   const fieldType = field?.type;
-  if (field.keyType) {
+  if (field?.keyType) {
     if (field.keyType !== 'string')
       return Error(`${field.keyType} map keys not supported`);
-    // TODO: Check that firestoreValue is an object.
+    if (typeof firestoreValue !== 'object' || Array.isArray(firestoreValue))
+      return Error(`Expected object, but got ${firestoreValue}`);
     return toMapValue(messageTypePath, fieldType, firestoreValue);
   } else if (field?.rule === 'repeated') {
     if (!Array.isArray(firestoreValue))
@@ -96,6 +97,7 @@ function toMapValue(
   valueType: string,
   nestedObject: [key: string]
 ): any | null {
+  if (!nestedObject) return null;
   const messageMap: { [key: string]: any } = {};
   for (const key in nestedObject) {
     const firestoreValue = nestedObject[key];
@@ -149,8 +151,7 @@ function toMessageOrEnumValue(
     ? registry.getMessageDescriptor(constructor)
     : null;
   if (constructor && nestedDescriptor) {
-    if (!nestedDescriptor)
-      return Error(`Unknown message type ${nestedDescriptor}`);
+    if (firestoreValue === null || firestoreValue === undefined) return null;
     // TODO: Check firestoreValue is a DocumentData.
     return toMessageInternal(
       firestoreValue as DocumentData,