diff --git a/js/coins/view/CoinMeasurementHistogram.ts b/js/coins/view/CoinMeasurementHistogram.ts
index adbf020..52644bb 100644
--- a/js/coins/view/CoinMeasurementHistogram.ts
+++ b/js/coins/view/CoinMeasurementHistogram.ts
@@ -60,9 +60,11 @@ export default class CoinMeasurementHistogram extends QuantumMeasurementHistogra
 
     // Create the number Properties for the left and right histogram bars.
     const leftNumberProperty = new NumberProperty( 0, {
+      phetioReadOnly: true,
       tandem: providedOptions.tandem.createTandem( 'leftNumberProperty' )
     } );
     const rightNumberProperty = new NumberProperty( 0, {
+      phetioReadOnly: true,
       tandem: providedOptions.tandem.createTandem( 'rightNumberProperty' )
     } );