diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index d8cfa324f9..6075f09b51 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -263,6 +263,10 @@ function size_format( $bytes, $decimals = 0 ) {
 		'B'  => 1,
 	);
 
+	if ( 0 === $bytes ) {
+		return number_format_i18n( 0, $decimals ) . ' B';
+	}
+
 	foreach ( $quant as $unit => $mag ) {
 		if ( doubleval( $bytes ) >= $mag ) {
 			return number_format_i18n( $bytes / $mag, $decimals ) . ' ' . $unit;
diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index e28e65f752..ce5f6579f2 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -51,28 +51,6 @@ class Tests_Functions extends WP_UnitTestCase {
 		$this->assertInternalType( 'string', $args['bar'] );
 	}
 
-	function test_size_format() {
-		$b  = 1;
-		$kb = 1024;
-		$mb = $kb*1024;
-		$gb = $mb*1024;
-		$tb = $gb*1024;
-		// test if boundaries are correct
-		$this->assertEquals('1 GB', size_format($gb, 0));
-		$this->assertEquals('1 MB', size_format($mb, 0));
-		$this->assertEquals('1 KB', size_format($kb, 0));
-		$this->assertEquals('1 B',  size_format($b, 0));
-		// now some values around
-		// add some bytes to make sure the result isn't 1.4999999
-		$this->assertEquals('1.5 TB', size_format($tb + $tb/2 + $mb, 1));
-		$this->assertEquals('1,023.999 GB', size_format($tb-$mb-$kb, 3));
-		// edge
-		$this->assertFalse(size_format(-1));
-		$this->assertFalse(size_format(0));
-		$this->assertFalse(size_format('baba'));
-		$this->assertFalse(size_format(array()));
-	}
-
 	/**
 	 * @ticket 35972
 	 */
diff --git a/tests/phpunit/tests/functions/sizeFormat.php b/tests/phpunit/tests/functions/sizeFormat.php
new file mode 100644
index 0000000000..bc01cb740e
--- /dev/null
+++ b/tests/phpunit/tests/functions/sizeFormat.php
@@ -0,0 +1,52 @@
+<?php
+
+/**
+ * Tests for size_format()
+ *
+ * @group functions.php
+ * @ticket 36635
+ */
+class Tests_Functions_Size_Format extends WP_UnitTestCase {
+	public function _data_size_format() {
+		return array(
+			array( array(), 0, false ),
+			array( 'baba', 0, false ),
+			array( '', 0, false ),
+			array( '-1', 0, false ),
+			array( -1, 0, false ),
+			array( 0, 0, '0 B' ),
+			array( 1, 0, '1 B' ),
+			array( 1023, 0, '1,023 B' ),
+			array( KB_IN_BYTES, 0, '1 KB' ),
+			array( KB_IN_BYTES, 2, '1.00 KB' ),
+			array( 2.5 * KB_IN_BYTES, 0, '3 KB' ),
+			array( 2.5 * KB_IN_BYTES, 2, '2.50 KB' ),
+			array( 10 * KB_IN_BYTES, 0, '10 KB' ),
+			array( (string) 1024 * KB_IN_BYTES, 2, '1.00 MB' ),
+			array( MB_IN_BYTES, 0, '1 MB' ),
+			array( 2.5 * MB_IN_BYTES, 0, '3 MB' ),
+			array( 2.5 * MB_IN_BYTES, 2, '2.50 MB' ),
+			array( (string) 1024 * MB_IN_BYTES, 2, '1.00 GB' ),
+			array( GB_IN_BYTES, 0, '1 GB' ),
+			array( 2.5 * GB_IN_BYTES, 0, '3 GB' ),
+			array( 2.5 * GB_IN_BYTES, 2, '2.50 GB' ),
+			array( (string) 1024 * GB_IN_BYTES, 2, '1.00 TB' ),
+			array( TB_IN_BYTES, 0, '1 TB' ),
+			array( 2.5 * TB_IN_BYTES, 0, '3 TB' ),
+			array( 2.5 * TB_IN_BYTES, 2, '2.50 TB' ),
+			array( TB_IN_BYTES + (TB_IN_BYTES/2) + MB_IN_BYTES, 1, '1.5 TB' ),
+			array( TB_IN_BYTES - MB_IN_BYTES - KB_IN_BYTES, 3, '1,023.999 GB' ),
+		);
+	}
+
+	/**
+	 * @dataProvider _data_size_format
+	 *
+	 * @param $bytes
+	 * @param $decimals
+	 * @param $expected
+	 */
+	public function test_size_format( $bytes, $decimals, $expected ) {
+		$this->assertSame( $expected, size_format( $bytes, $decimals ) );
+	}
+}