diff --git a/src/generate-generic-service-definition.ts b/src/generate-generic-service-definition.ts
index c9a0e1723..963fd9d9e 100644
--- a/src/generate-generic-service-definition.ts
+++ b/src/generate-generic-service-definition.ts
@@ -88,13 +88,14 @@ function generateMethodOptions(ctx: Context, options: MethodOptions | undefined)
       chunks.push(code`idempotencyLevel: 'NO_SIDE_EFFECTS',`);
     }
 
-    if (options._unknownFields !== undefined) {
+    if ("_unknownFields" in options) {
+      const msgUnknownFields: any = (options as any)["_unknownFields"];
       const unknownFieldsChunks: Code[] = [];
 
       unknownFieldsChunks.push(code`{`);
 
-      for (const key in options._unknownFields) {
-        const values = options._unknownFields[key];
+      for (const key of Object.keys(msgUnknownFields)) {
+        const values = msgUnknownFields[key] as Uint8Array[];
         const valuesChunks: Code[] = [];
 
         for (const value of values) {
diff --git a/src/schema.ts b/src/schema.ts
index 085438bb7..0e6f7c49d 100644
--- a/src/schema.ts
+++ b/src/schema.ts
@@ -101,8 +101,12 @@ export function generateSchema(ctx: Context, fileDesc: FileDescriptorProto, sour
 
   let fileOptions: Code | undefined;
   if (fileDesc.options) {
-    fileOptions = encodedOptionsToOptions(ctx, ".google.protobuf.FileOptions", fileDesc.options._unknownFields);
-    delete fileDesc.options._unknownFields;
+    fileOptions = encodedOptionsToOptions(
+      ctx,
+      ".google.protobuf.FileOptions",
+      (fileDesc.options as any)["_unknownFields"]
+    );
+    delete (fileDesc.options as any)["_unknownFields"];
   }
 
   const messagesOptions: Code[] = [];
@@ -121,9 +125,9 @@ export function generateSchema(ctx: Context, fileDesc: FileDescriptorProto, sour
         const methodOptions = encodedOptionsToOptions(
           ctx,
           ".google.protobuf.MethodOptions",
-          method.options._unknownFields
+          (method.options as any)["_unknownFields"]
         );
-        delete method.options._unknownFields;
+        delete (method.options as any)["_unknownFields"];
         if (methodOptions) {
           methodsOptions.push(code`'${method.name}': ${methodOptions}`);
         }
@@ -132,8 +136,12 @@ export function generateSchema(ctx: Context, fileDesc: FileDescriptorProto, sour
 
     let serviceOptions: Code | undefined;
     if (service.options) {
-      serviceOptions = encodedOptionsToOptions(ctx, ".google.protobuf.ServiceOptions", service.options._unknownFields);
-      delete service.options._unknownFields;
+      serviceOptions = encodedOptionsToOptions(
+        ctx,
+        ".google.protobuf.ServiceOptions",
+        (service.options as any)["_unknownFields"]
+      );
+      delete (service.options as any)["_unknownFields"];
     }
 
     if (methodsOptions.length > 0 || serviceOptions) {
@@ -154,9 +162,9 @@ export function generateSchema(ctx: Context, fileDesc: FileDescriptorProto, sour
         const valueOptions = encodedOptionsToOptions(
           ctx,
           ".google.protobuf.EnumValueOptions",
-          value.options._unknownFields
+          (value.options as any)["_unknownFields"]
         );
-        delete value.options._unknownFields;
+        delete (value.options as any)["_unknownFields"];
         if (valueOptions) {
           valuesOptions.push(code`'${value.name}': ${valueOptions}`);
         }
@@ -165,8 +173,12 @@ export function generateSchema(ctx: Context, fileDesc: FileDescriptorProto, sour
 
     let enumOptions: Code | undefined;
     if (Enum.options) {
-      enumOptions = encodedOptionsToOptions(ctx, ".google.protobuf.EnumOptions", Enum.options._unknownFields);
-      delete Enum.options._unknownFields;
+      enumOptions = encodedOptionsToOptions(
+        ctx,
+        ".google.protobuf.EnumOptions",
+        (Enum.options as any)["_unknownFields"]
+      );
+      delete (Enum.options as any)["_unknownFields"];
     }
 
     if (valuesOptions.length > 0 || enumOptions) {
@@ -227,14 +239,13 @@ function getExtensionValue(ctx: Context, extension: FieldDescriptorProto, data:
 function encodedOptionsToOptions(
   ctx: Context,
   extendee: string,
-  encodedOptions?: { [key: number]: Uint8Array[] }
+  encodedOptions: { [key: number]: Uint8Array[] }
 ): Code | undefined {
   if (!encodedOptions) {
     return undefined;
   }
   const resultOptions: Code[] = [];
-  for (const key in encodedOptions) {
-    const value = encodedOptions[key];
+  for (const [key, value] of Object.entries(encodedOptions)) {
     const extension = extensionCache[extendee][parseInt(key, 10) >>> 3];
     resultOptions.push(getExtensionValue(ctx, extension, value));
   }
@@ -248,8 +259,12 @@ function resolveMessageOptions(ctx: Context, message: DescriptorProto): Code | u
   const fieldsOptions: Code[] = [];
   message.field.forEach((field) => {
     if (field.options) {
-      const fieldOptions = encodedOptionsToOptions(ctx, ".google.protobuf.FieldOptions", field.options._unknownFields);
-      delete field.options._unknownFields;
+      const fieldOptions = encodedOptionsToOptions(
+        ctx,
+        ".google.protobuf.FieldOptions",
+        (field.options as any)["_unknownFields"]
+      );
+      delete (field.options as any)["_unknownFields"];
       if (fieldOptions) {
         fieldsOptions.push(code`'${field.name}': ${fieldOptions}`);
       }
@@ -259,8 +274,12 @@ function resolveMessageOptions(ctx: Context, message: DescriptorProto): Code | u
   const oneOfsOptions: Code[] = [];
   message.oneofDecl.forEach((oneOf) => {
     if (oneOf.options) {
-      const oneOfOptions = encodedOptionsToOptions(ctx, ".google.protobuf.OneofOptions", oneOf.options._unknownFields);
-      delete oneOf.options._unknownFields;
+      const oneOfOptions = encodedOptionsToOptions(
+        ctx,
+        ".google.protobuf.OneofOptions",
+        (oneOf.options as any)["_unknownFields"]
+      );
+      delete (oneOf.options as any)["_unknownFields"];
       if (oneOfOptions) {
         oneOfsOptions.push(code`'${oneOf.name}': ${oneOfOptions}`);
       }
@@ -279,8 +298,12 @@ function resolveMessageOptions(ctx: Context, message: DescriptorProto): Code | u
 
   let messageOptions: Code | undefined;
   if (message.options) {
-    messageOptions = encodedOptionsToOptions(ctx, ".google.protobuf.MessageOptions", message.options._unknownFields);
-    delete message.options._unknownFields;
+    messageOptions = encodedOptionsToOptions(
+      ctx,
+      ".google.protobuf.MessageOptions",
+      (message.options as any)["_unknownFields"]
+    );
+    delete (message.options as any)["_unknownFields"];
   }
 
   if (fieldsOptions.length > 0 || oneOfsOptions.length > 0 || nestedOptions.length > 0 || messageOptions) {