diff --git a/CHANGELOG-GRAVITYPDF.md b/CHANGELOG-GRAVITYPDF.md
index bbfff2e9f..d4b40a2a9 100644
--- a/CHANGELOG-GRAVITYPDF.md
+++ b/CHANGELOG-GRAVITYPDF.md
@@ -273,6 +273,7 @@ Bugfixes
 * Arabic in an SVG was not given an Arabic font. [#281]
 * An RGB image converted to CMYK truncated its inks, so full ink fell short of 255. [#349] [#354]
 * An image's `/SMask` pointed at whichever object came before it, which is only its mask if nothing was added in between. An image and its soft mask are now registered together. [#373]
+* A `<textcircle>` with no `font-size` from its tag or the CSS raised three `Undefined array key "fontsize"` warnings. It is now drawn at the size of the text around it, as though that size were set on it. [#645] [#678]
 
 ### Forms
 
@@ -935,3 +936,5 @@ These changes do not change output.
 [#543]: https://github.com/GravityPDF/mpdf/issues/543
 [#656]: https://github.com/GravityPDF/mpdf/pull/656
 [#654]: https://github.com/GravityPDF/mpdf/pull/654
+[#645]: https://github.com/GravityPDF/mpdf/issues/645
+[#678]: https://github.com/GravityPDF/mpdf/pull/678
diff --git a/src/Tag/TextCircle.php b/src/Tag/TextCircle.php
index 6e5652f57..bcef38d5a 100644
--- a/src/Tag/TextCircle.php
+++ b/src/Tag/TextCircle.php
@@ -74,6 +74,7 @@ public function open($attr, &$ahtml, &$ihtml)
 				$objattr['visibility'] = $v;
 			}
 		}
+		$objattr['fontsize'] = $this->mpdf->FontSizePt;
 		if (isset($properties['FONT-SIZE'])) {
 			if (strtolower($properties['FONT-SIZE']) === 'auto') {
 				if ($objattr['top-text'] && $objattr['bottom-text']) {
diff --git a/tests/Mpdf/TextCircleFontSizeTest.php b/tests/Mpdf/TextCircleFontSizeTest.php
new file mode 100644
index 000000000..4d0770a7c
--- /dev/null
+++ b/tests/Mpdf/TextCircleFontSizeTest.php
@@ -0,0 +1,66 @@
+<?php
+
+namespace Mpdf;
+
+/**
+ * A text circle that neither its tag nor the CSS gives a font-size is drawn at the size of the text around it, as
+ * though that size had been set on it, rather than raising warnings over the size it was never given
+ */
+class TextCircleFontSizeTest extends \Yoast\PHPUnitPolyfills\TestCases\TestCase
+{
+
+	use PageStreams;
+
+	/**
+	 * @dataProvider providerContexts
+	 *
+	 * @param string $mode A CssMode constant
+	 * @param string $html The document, with {style} where the text circle's style attribute goes
+	 * @param string $size The size around the text circle, as a Tf operator writes it
+	 */
+	public function testATextCircleWithoutAFontSizeDrawsAtTheSizeAroundIt($mode, $html, $size)
+	{
+		$pages = $this->drawnPages($mode, $html, '');
+		$this->assertStringContainsString(' ' . $size . ' Tf', $pages[0]);
+		$this->assertSame($this->drawnPages($mode, $html, ' style="font-size: ' . $size . 'pt"'), $pages);
+	}
+
+	/**
+	 * Draw the document, asserting it raises nothing, and hand back its page content streams
+	 *
+	 * @param string $mode A CssMode constant
+	 * @param string $html The document, with {style} where the text circle's style attribute goes
+	 * @param string $style What goes in place of {style}
+	 *
+	 * @return string[]
+	 */
+	private function drawnPages($mode, $html, $style)
+	{
+		$pdf = $this->assertDrawsSilently(function (Mpdf $mpdf) use ($html, $style) {
+			$mpdf->WriteHTML(str_replace('{style}', $style, $html));
+		}, ['cssMode' => $mode]);
+
+		return $this->pages($pdf);
+	}
+
+	/**
+	 * @return array[] Each CSS mode with a text circle at the top of the document, in a paragraph and in a table cell
+	 */
+	public function providerContexts()
+	{
+		$contexts = [
+			'document' => ['<textcircle r="30mm" top-text="Top" bottom-text="Bottom"{style} />', '11.000'],
+			'paragraph' => ['<p style="font-size: 14pt"><textcircle r="30mm" top-text="Top"{style} /></p>', '14.000'],
+			'table cell' => ['<table><tr><td style="font-size: 9pt"><textcircle r="10mm" top-text="Top"{style} /></td></tr></table>', '9.000'],
+		];
+
+		$cases = [];
+		foreach ([CssMode::STANDARD, CssMode::LEGACY] as $mode) {
+			foreach ($contexts as $name => $context) {
+				$cases[$mode . ', ' . $name] = array_merge([$mode], $context);
+			}
+		}
+
+		return $cases;
+	}
+}