diff --git a/components/identity/identity-form.tsx b/components/identity/identity-form.tsx
index 1dbefb65..30b1898f 100644
--- a/components/identity/identity-form.tsx
+++ b/components/identity/identity-form.tsx
@@ -36,10 +36,10 @@ function truncateToUtf8Bytes(s: string, maxBytes: number): string {
 interface IdentityFormData {
   name: string;
   email: string;
-  replyTo?: EmailAddress[];
-  bcc?: EmailAddress[];
-  textSignature?: string;
-  htmlSignature?: string;
+  replyTo?: EmailAddress[] | null;
+  bcc?: EmailAddress[] | null;
+  textSignature?: string | null;
+  htmlSignature?: string | null;
 }
 
 interface IdentityFormProps {
@@ -121,14 +121,16 @@ export function IdentityForm({ identity, onSave, onCancel }: IdentityFormProps)
     setIsSubmitting(true);
 
     try {
-      // Sanitize HTML signature before sending to server
+      // JMAP needs explicit null to clear a field; undefined would be dropped
+      // from the JSON payload and leave the server-side value untouched.
+      const trimmedText = formData.textSignature?.trim() ?? '';
+      const trimmedHtml = formData.htmlSignature?.trim() ?? '';
       const sanitizedData: IdentityFormData = {
         ...formData,
-        replyTo: parseEmailList(replyToInput),
-        bcc: parseEmailList(bccInput),
-        htmlSignature: formData.htmlSignature
-          ? sanitizeSignatureHtml(formData.htmlSignature)
-          : undefined,
+        textSignature: trimmedText ? formData.textSignature : null,
+        htmlSignature: trimmedHtml ? sanitizeSignatureHtml(formData.htmlSignature!) : null,
+        replyTo: parseEmailList(replyToInput) ?? null,
+        bcc: parseEmailList(bccInput) ?? null,
       };
 
       await onSave(sanitizedData);
@@ -271,7 +273,7 @@ export function IdentityForm({ identity, onSave, onCancel }: IdentityFormProps)
         </label>
         <textarea
           id="identity-text-sig"
-          value={formData.textSignature}
+          value={formData.textSignature ?? ''}
           onChange={(e) => setFormData({ ...formData, textSignature: truncateToUtf8Bytes(e.target.value, SIGNATURE_MAX_BYTES) })}
           rows={3}
           disabled={isSubmitting}
@@ -289,7 +291,7 @@ export function IdentityForm({ identity, onSave, onCancel }: IdentityFormProps)
         </label>
         <textarea
           id="identity-html-sig"
-          value={formData.htmlSignature}
+          value={formData.htmlSignature ?? ''}
           onChange={(e) => setFormData({ ...formData, htmlSignature: truncateToUtf8Bytes(e.target.value, SIGNATURE_MAX_BYTES) })}
           rows={5}
           disabled={isSubmitting}
diff --git a/components/identity/identity-manager-modal.tsx b/components/identity/identity-manager-modal.tsx
index ea6a916a..59ef035d 100644
--- a/components/identity/identity-manager-modal.tsx
+++ b/components/identity/identity-manager-modal.tsx
@@ -32,10 +32,10 @@ function emailMatchesUsername(email: string, username: string): boolean {
 interface IdentityFormData {
   name: string;
   email: string;
-  replyTo?: EmailAddress[];
-  bcc?: EmailAddress[];
-  textSignature?: string;
-  htmlSignature?: string;
+  replyTo?: EmailAddress[] | null;
+  bcc?: EmailAddress[] | null;
+  textSignature?: string | null;
+  htmlSignature?: string | null;
 }
 
 interface IdentityManagerModalProps {
diff --git a/lib/demo/demo-client.ts b/lib/demo/demo-client.ts
index 5b825a94..1fd0bdbb 100644
--- a/lib/demo/demo-client.ts
+++ b/lib/demo/demo-client.ts
@@ -530,7 +530,7 @@ export class DemoJMAPClient implements IJMAPClient {
   async createIdentity(
     name: string, email: string,
     replyTo?: EmailAddress[] | null, bcc?: EmailAddress[] | null,
-    htmlSignature?: string, textSignature?: string,
+    textSignature?: string | null, htmlSignature?: string | null,
   ): Promise<Identity> {
     const identity: Identity = {
       id: generateDemoId('identity'), name, email,
@@ -542,9 +542,14 @@ export class DemoJMAPClient implements IJMAPClient {
     return identity;
   }
 
-  async updateIdentity(identityId: string, updates: { name?: string; replyTo?: EmailAddress[] | null; bcc?: EmailAddress[] | null; htmlSignature?: string; textSignature?: string }): Promise<void> {
+  async updateIdentity(identityId: string, updates: { name?: string | null; replyTo?: EmailAddress[] | null; bcc?: EmailAddress[] | null; textSignature?: string | null; htmlSignature?: string | null }): Promise<void> {
     const identity = this.data.identities.find(i => i.id === identityId);
-    if (identity) Object.assign(identity, updates);
+    if (!identity) return;
+    if (updates.name !== undefined) identity.name = updates.name ?? '';
+    if (updates.replyTo !== undefined) identity.replyTo = updates.replyTo ?? undefined;
+    if (updates.bcc !== undefined) identity.bcc = updates.bcc ?? undefined;
+    if (updates.textSignature !== undefined) identity.textSignature = updates.textSignature ?? '';
+    if (updates.htmlSignature !== undefined) identity.htmlSignature = updates.htmlSignature ?? '';
   }
 
   async deleteIdentity(identityId: string): Promise<void> {
diff --git a/lib/jmap/client-interface.ts b/lib/jmap/client-interface.ts
index e3c0ca74..1ccbbd5f 100644
--- a/lib/jmap/client-interface.ts
+++ b/lib/jmap/client-interface.ts
@@ -183,17 +183,17 @@ export interface IJMAPClient {
     email: string,
     replyTo?: EmailAddress[] | null,
     bcc?: EmailAddress[] | null,
-    htmlSignature?: string,
-    textSignature?: string,
+    textSignature?: string | null,
+    htmlSignature?: string | null,
   ): Promise<Identity>;
   updateIdentity(
     identityId: string,
     updates: {
-      name?: string;
+      name?: string | null;
       replyTo?: EmailAddress[] | null;
       bcc?: EmailAddress[] | null;
-      htmlSignature?: string;
-      textSignature?: string;
+      textSignature?: string | null;
+      htmlSignature?: string | null;
     },
   ): Promise<void>;
   deleteIdentity(identityId: string): Promise<void>;
diff --git a/lib/jmap/client.ts b/lib/jmap/client.ts
index 4818de12..f32a95db 100644
--- a/lib/jmap/client.ts
+++ b/lib/jmap/client.ts
@@ -1813,10 +1813,10 @@ export class JMAPClient implements IJMAPClient {
   async createIdentity(
     name: string,
     email: string,
-    replyTo?: EmailAddress[],
-    bcc?: EmailAddress[],
-    textSignature?: string,
-    htmlSignature?: string
+    replyTo?: EmailAddress[] | null,
+    bcc?: EmailAddress[] | null,
+    textSignature?: string | null,
+    htmlSignature?: string | null
   ): Promise<Identity> {
     const response = await this.request([
       ["Identity/set", {
@@ -1859,11 +1859,11 @@ export class JMAPClient implements IJMAPClient {
   async updateIdentity(
     identityId: string,
     updates: {
-      name?: string;
-      replyTo?: EmailAddress[];
-      bcc?: EmailAddress[];
-      textSignature?: string;
-      htmlSignature?: string;
+      name?: string | null;
+      replyTo?: EmailAddress[] | null;
+      bcc?: EmailAddress[] | null;
+      textSignature?: string | null;
+      htmlSignature?: string | null;
     }
   ): Promise<void> {
     const response = await this.request([