diff --git a/includes/fields/class-acf-field-checkbox.php b/includes/fields/class-acf-field-checkbox.php
index 5935a041..abd434c8 100644
--- a/includes/fields/class-acf-field-checkbox.php
+++ b/includes/fields/class-acf-field-checkbox.php
@@ -499,22 +499,34 @@ function update_value( $value, $post_id, $field ) {
 			// select -> update_value()
 			$value = acf_get_field_type( 'select' )->update_value( $value, $post_id, $field );
 
-			// save_other_choice
-			if ( $field['save_custom'] ) {
+			// save_custom
+			if ( ! empty( $field['save_custom'] ) && is_array( $value ) ) {
 
 				// get raw $field (may have been changed via repeater field)
 				// if field is local, it won't have an ID
-				$selector = $field['ID'] ? $field['ID'] : $field['key'];
-				$field    = acf_get_field( $selector );
-				if ( ! $field ) {
-					return false;
+				$selector = '';
+				if ( ! empty( $field['ID'] ) ) {
+					$selector = $field['ID'];
+				} elseif ( ! empty( $field['key'] ) ) {
+					$selector = $field['key'];
 				}
 
-				// bail early if no ID (JSON only)
-				if ( ! $field['ID'] ) {
+				// bail early if we have no selector for lookup
+				if ( ! $selector ) {
 					return $value;
 				}
 
+				$field = acf_get_field( $selector );
+
+				// bail early if lookup failed or field is JSON only
+				if ( ! is_array( $field ) || empty( $field['ID'] ) ) {
+					return $value;
+				}
+
+				if ( ! isset( $field['choices'] ) || ! is_array( $field['choices'] ) ) {
+					$field['choices'] = array();
+				}
+
 				// loop
 				foreach ( $value as $v ) {
 
diff --git a/includes/fields/class-acf-field-select.php b/includes/fields/class-acf-field-select.php
index bd9f810b..5fb07f2e 100644
--- a/includes/fields/class-acf-field-select.php
+++ b/includes/fields/class-acf-field-select.php
@@ -576,14 +576,29 @@ public function update_value( $value, $post_id, $field ) {
 			// Save custom options back to the field definition if configured.
 			if ( ! empty( $field['save_options'] ) && is_array( $value ) ) {
 				// Get the raw field, using the ID if present or the key otherwise (i.e. when using JSON).
-				$selector = $field['ID'] ? $field['ID'] : $field['key'];
-				$field    = acf_get_field( $selector );
+				$selector = '';
+				if ( ! empty( $field['ID'] ) ) {
+					$selector = $field['ID'];
+				} elseif ( ! empty( $field['key'] ) ) {
+					$selector = $field['key'];
+				}
+
+				// Bail when field lookup can't be attempted.
+				if ( ! $selector ) {
+					return $value;
+				}
+
+				$field = acf_get_field( $selector );
 
 				// Bail if we don't have a valid field or field ID (JSON only).
-				if ( empty( $field['ID'] ) ) {
+				if ( ! is_array( $field ) || empty( $field['ID'] ) ) {
 					return $value;
 				}
 
+				if ( ! isset( $field['choices'] ) || ! is_array( $field['choices'] ) ) {
+					$field['choices'] = array();
+				}
+
 				foreach ( $value as $v ) {
 					// Ignore if the option already exists.
 					if ( isset( $field['choices'][ $v ] ) ) {
diff --git a/tests/php/includes/fields/test-class-acf-field-checkbox.php b/tests/php/includes/fields/test-class-acf-field-checkbox.php
index e46c28e1..63456bf7 100644
--- a/tests/php/includes/fields/test-class-acf-field-checkbox.php
+++ b/tests/php/includes/fields/test-class-acf-field-checkbox.php
@@ -176,6 +176,35 @@ public function test_update_value_empty() {
 		$this->assertEmpty( $result );
 	}
 
+	/**
+	 * Test save_custom does not trigger warnings for JSON-only fields.
+	 */
+	public function test_update_value_save_custom_handles_json_field_without_id() {
+		$field = $this->get_field(
+			array(
+				'save_custom' => 1,
+			)
+		);
+
+		$result = $this->field_instance->update_value( array( 'custom_value' ), $this->post_id, $field );
+		$this->assertSame( array( 'custom_value' ), $result );
+	}
+
+	/**
+	 * Test save_custom bails early when field key and ID are both missing.
+	 */
+	public function test_update_value_save_custom_handles_missing_field_identifier() {
+		$field = $this->get_field(
+			array(
+				'save_custom' => 1,
+			)
+		);
+		unset( $field['key'] );
+
+		$result = $this->field_instance->update_value( array( 'custom_value' ), $this->post_id, $field );
+		$this->assertSame( array( 'custom_value' ), $result );
+	}
+
 	/**
 	 * Test validate_value with valid selection.
 	 */
diff --git a/tests/php/includes/fields/test-class-acf-field-select.php b/tests/php/includes/fields/test-class-acf-field-select.php
index 106b7969..4e0c9b28 100644
--- a/tests/php/includes/fields/test-class-acf-field-select.php
+++ b/tests/php/includes/fields/test-class-acf-field-select.php
@@ -202,6 +202,37 @@ public function test_update_value_converts_to_strings() {
 		$this->assertContains( 'blue', $result );
 	}
 
+	/**
+	 * Test save_options does not trigger warnings for JSON-only fields.
+	 */
+	public function test_update_value_save_options_handles_json_field_without_id() {
+		$field = $this->get_field(
+			array(
+				'save_options' => 1,
+				'multiple'     => 1,
+			)
+		);
+
+		$result = $this->field_instance->update_value( array( 'custom_value' ), $this->post_id, $field );
+		$this->assertSame( array( 'custom_value' ), $result );
+	}
+
+	/**
+	 * Test save_options bails early when field key and ID are both missing.
+	 */
+	public function test_update_value_save_options_handles_missing_field_identifier() {
+		$field = $this->get_field(
+			array(
+				'save_options' => 1,
+				'multiple'     => 1,
+			)
+		);
+		unset( $field['key'] );
+
+		$result = $this->field_instance->update_value( array( 'custom_value' ), $this->post_id, $field );
+		$this->assertSame( array( 'custom_value' ), $result );
+	}
+
 	/**
 	 * Test get_rest_schema returns valid schema.
 	 */