@@ -21,6 +21,7 @@
 -   `BlockCompare`: Show whitespace differences in the Resolve Block dialog. The markup's spaces and blank lines are kept instead of collapsed by the browser, and each added or removed line break gets a visible marker, so a diff made only of whitespace no longer looks identical on both sides. The dialog also shows the markup of a converted Custom HTML block, which is kept in its `innerContent` rather than produced by `save`, instead of an empty diff ([#82397](https://github.com/WordPress/gutenberg/pull/82397)).
 -   Inserter: Stop re-rendering the block list when the block selection changes while the inserter is open ([#83284](https://github.com/WordPress/gutenberg/pull/83284)).
 -   Fix style edits applied to a multi-block selection overwriting each block's other styles. Only the changed values are now applied to every selected block, so distinct styles are preserved and resets propagate to the whole selection ([#80070](https://github.com/WordPress/gutenberg/pull/80070)).
+-   Fix exception thrown by `getSelectedBlockStyleState` when an `undefined` `clientId` is passed as an argument. ([#84140](https://github.com/WordPress/gutenberg/pull/84140))
 
 ### Internal
 
 
@@ -219,9 +219,9 @@ function BlockInspector() {
 			hasBlockStyles: _hasBlockStyles,
 			editedContentOnlySection: getEditedContentOnlySection(),
 			blockEditingMode: getBlockEditingMode( _renderedBlockClientId ),
-			selectedBlockStyleState: getSelectedBlockStyleState(
-				_renderedBlockClientId
-			),
+			selectedBlockStyleState: _renderedBlockClientId
+				? getSelectedBlockStyleState( _renderedBlockClientId )
+				: undefined,
 			showStateOnCanvas: isSelectedBlockStyleStateShownOnCanvas(
 				_renderedBlockClientId
 			),
 
@@ -133,6 +133,7 @@ export function PrivateBlockToolbar( {
 				isBlockHiddenAtViewport( id, _currentDeviceType )
 			);
 		const _isEditingResponsiveStyleState =
+			!! selectedBlockClientId &&
 			isResponsiveEditing() &&
 			hasViewportBlockStyleState(
 				getSelectedBlockStyleState( selectedBlockClientId )
 
@@ -3629,8 +3629,8 @@ export function __unstableGetTemporarilyEditingAsBlocks( state ) {
 export const getSelectedBlockStyleState = createSelector(
 	( state, clientId ) => {
 		const perBlockState =
-			state.selectedBlockStyleState?.clientId === clientId
-				? ( state.selectedBlockStyleState.value ??
+			clientId && state.selectedBlockStyleState?.clientId === clientId
+				? ( state.selectedBlockStyleState?.value ??
 					DEFAULT_BLOCK_STYLE_STATE )
 				: DEFAULT_BLOCK_STYLE_STATE;
 
 
@@ -5581,6 +5581,15 @@ describe( 'getBlockEditingMode', () => {
 	} );
 
 	describe( 'getSelectedBlockStyleState', () => {
+		it( 'returns default when no clientId is passed', () => {
+			const state = {};
+
+			expect( getSelectedBlockStyleState( state, undefined ) ).toEqual( {
+				viewport: 'default',
+				pseudo: 'default',
+			} );
+		} );
+
 		it( 'returns default when the block has no selected state', () => {
 			const state = {};