diff -Naur mantisbt-1.2.3/bug_report.php mantisbt-patch/bug_report.php
--- mantisbt-1.2.3/bug_report.php	2010-09-27 16:11:01.000000000 +0200
+++ mantisbt-patch/bug_report.php	2010-09-27 16:09:38.000000000 +0200
@@ -104,18 +104,24 @@
 		$t_def = custom_field_get_definition( $t_id );
 
 		# Produce an error if the field is required but wasn't posted
-		if ( !gpc_isset_custom_field( $t_id, $t_def['type'] ) &&
-			( $t_def['require_report'] ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_ENUM ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_LIST ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_MULTILIST ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_RADIO ) ) {
+		if	(	$t_def['require_report']
+			&&	$t_def['type'] != CUSTOM_FIELD_TYPE_CHECKBOX
+			&&	!gpc_isset_custom_field($t_id, $t_def['type'])
+			) {
 			error_parameters( lang_get_defaulted( custom_field_get_field( $t_id, 'name' ) ) );
 			trigger_error( ERROR_EMPTY_FIELD, ERROR );
 		}
-		if ( !custom_field_validate( $t_id, gpc_get_custom_field( "custom_field_$t_id", $t_def['type'], NULL ) ) ) {
-			error_parameters( lang_get_defaulted( custom_field_get_field( $t_id, 'name' ) ) );
-			trigger_error( ERROR_CUSTOM_FIELD_INVALID_VALUE, ERROR );
+		if(gpc_isset_custom_field($t_id, $t_def['type'])) {
+			$t_custom_field_value = gpc_get_custom_field("custom_field_$t_id", $t_def['type'], NULL);
+			if	(	$t_def['display_report']
+				&&	!custom_field_validate(
+							$t_id,
+							$t_custom_field_value
+						) 
+				) {
+				error_parameters( lang_get_defaulted( custom_field_get_field( $t_id, 'name' ) ) );
+				trigger_error( ERROR_CUSTOM_FIELD_INVALID_VALUE, ERROR );
+			}
 		}
 	}
 
@@ -135,13 +141,21 @@
 
 	# Handle custom field submission
 	foreach( $t_related_custom_field_ids as $t_id ) {
+		$t_def = custom_field_get_definition( $t_id );
 		# Do not set custom field value if user has no write access.
-		if( !custom_field_has_write_access( $t_id, $t_bug_id ) ) {
+		if	(	!$t_def['display_report']
+			||	!custom_field_has_write_access( $t_id, $t_bug_id )
+			) {
 			continue;
 		}
 
-		$t_def = custom_field_get_definition( $t_id );
-		if( !custom_field_set_value( $t_id, $t_bug_id, gpc_get_custom_field( "custom_field_$t_id", $t_def['type'], '' ), false ) ) {
+		$t_custom_field_value = gpc_get_custom_field( "custom_field_$t_id", $t_def['type'], '');
+		if	(	empty($t_custom_field_value)
+			&&	!$t_def['require_report']
+			) {
+			continue;
+		}
+		if(!custom_field_set_value($t_id, $t_bug_id, $t_custom_field_value, false)) {
 			error_parameters( lang_get_defaulted( custom_field_get_field( $t_id, 'name' ) ) );
 			trigger_error( ERROR_CUSTOM_FIELD_INVALID_VALUE, ERROR );
 		}
diff -Naur mantisbt-1.2.3/bug_update.php mantisbt-patch/bug_update.php
--- mantisbt-1.2.3/bug_update.php	2010-09-27 16:10:52.000000000 +0200
+++ mantisbt-patch/bug_update.php	2010-09-27 16:09:33.000000000 +0200
@@ -137,19 +137,19 @@
 			continue;
 		}
 
+		$t_old_custom_field_value = custom_field_get_value( $t_id, $f_bug_id );
 		# Produce an error if the field is required but wasn't posted
-		if ( !gpc_isset_custom_field( $t_id, $t_def['type'] ) &&
-			( $t_def['require_' . $t_custom_status_label] ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_ENUM ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_LIST ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_MULTILIST ||
-				$t_def['type'] == CUSTOM_FIELD_TYPE_RADIO ) ) {
-			error_parameters( lang_get_defaulted( custom_field_get_field( $t_id, 'name' ) ) );
-			trigger_error( ERROR_EMPTY_FIELD, ERROR );
+		if	(	$t_def['require_' . $t_custom_status_label]
+			&&	$t_def['type'] != CUSTOM_FIELD_TYPE_CHECKBOX
+			&&	!gpc_isset_custom_field($t_id, $t_def['type'])
+			) {
+			if(!isset($t_old_custom_field_value)) {
+				error_parameters( lang_get_defaulted( custom_field_get_field( $t_id, 'name' ) ) );
+				trigger_error( ERROR_EMPTY_FIELD, ERROR );
+			}
 		}
 
 		$t_new_custom_field_value = gpc_get_custom_field( "custom_field_$t_id", $t_def['type'], '' );
-		$t_old_custom_field_value = custom_field_get_value( $t_id, $f_bug_id );
 
 		# Don't update the custom field if the new value both matches the old value and is valid
 		# This ensures that changes to custom field validation will force the update of old invalid custom field values
diff -Naur mantisbt-1.2.3/core/custom_field_api.php mantisbt-patch/core/custom_field_api.php
--- mantisbt-1.2.3/core/custom_field_api.php	2010-09-27 16:11:18.000000000 +0200
+++ mantisbt-patch/core/custom_field_api.php	2010-09-27 16:34:03.000000000 +0200
@@ -1172,12 +1172,11 @@
 			$t_valid &= ( $p_value == null ) || ( ( $p_value !== false ) && ( $p_value > 0 ) );
 			break;
 		case CUSTOM_FIELD_TYPE_CHECKBOX:
-			# Checkbox fields can hold a null value (when no checkboxes are ticked)
+		case CUSTOM_FIELD_TYPE_MULTILIST:
+			# These fields can hold a null value (when no values are selected)
 			if( $p_value === '' ) {
 				break;
 			}
-			# If checkbox field value is not null then we need to validate it... (note: no "break" statement here!)
-		case CUSTOM_FIELD_TYPE_MULTILIST:
 			$t_values = explode( '|', $p_value );
 			$t_possible_values = custom_field_prepare_possible_values( $row['possible_values'] );
 			$t_possible_values = explode( '|', $t_possible_values );