diff --git a/packages/block-editor/src/components/iframe/index.js b/packages/block-editor/src/components/iframe/index.js
index 293742f55683c5..a0a65e51dc0133 100644
--- a/packages/block-editor/src/components/iframe/index.js
+++ b/packages/block-editor/src/components/iframe/index.js
@@ -317,9 +317,11 @@ function Iframe( {
 						prevContainerWidthRef.current
 				: scale
 		);
+
+		// frameSize has to be a px value for the scaling and frame size to be computed correctly.
 		iframeDocument.documentElement.style.setProperty(
 			'--wp-block-editor-iframe-zoom-out-frame-size',
-			typeof frameSize === 'number' ? `${ frameSize }px` : frameSize
+			`${ frameSize }px`
 		);
 		iframeDocument.documentElement.style.setProperty(
 			'--wp-block-editor-iframe-zoom-out-content-height',