diff --git a/src/wp-includes/rest-api.php b/src/wp-includes/rest-api.php
index 5d8d34cd6b..8ad3cbc14a 100644
--- a/src/wp-includes/rest-api.php
+++ b/src/wp-includes/rest-api.php
@@ -1315,7 +1315,7 @@ function rest_is_boolean( $maybe_bool ) {
  * @return bool True if an integer, otherwise false.
  */
 function rest_is_integer( $maybe_integer ) {
-	return round( floatval( $maybe_integer ) ) === floatval( $maybe_integer );
+	return is_numeric( $maybe_integer ) && round( floatval( $maybe_integer ) ) === floatval( $maybe_integer );
 }
 
 /**
diff --git a/tests/phpunit/tests/rest-api.php b/tests/phpunit/tests/rest-api.php
index e9d9b50c5d..3a1240827b 100644
--- a/tests/phpunit/tests/rest-api.php
+++ b/tests/phpunit/tests/rest-api.php
@@ -1918,6 +1918,69 @@ class Tests_REST_API extends WP_UnitTestCase {
 		);
 	}
 
+	/**
+	 * @ticket 51146
+	 *
+	 * @dataProvider _dp_rest_is_integer
+	 *
+	 * @param bool  $expected Expected result of the check.
+	 * @param mixed $value    The value to check.
+	 */
+	public function test_rest_is_integer( $expected, $value ) {
+		$is_integer = rest_is_integer( $value );
+
+		if ( $expected ) {
+			$this->assertTrue( $is_integer );
+		} else {
+			$this->assertFalse( $is_integer );
+		}
+	}
+
+	public function _dp_rest_is_integer() {
+		return array(
+			array(
+				true,
+				1,
+			),
+			array(
+				true,
+				'1',
+			),
+			array(
+				true,
+				0,
+			),
+			array(
+				true,
+				-1,
+			),
+			array(
+				true,
+				'05',
+			),
+			array(
+				false,
+				'garbage',
+			),
+			array(
+				false,
+				5.5,
+			),
+			array(
+				false,
+				'5.5',
+			),
+			array(
+				false,
+				array(),
+			),
+			array(
+				false,
+				true,
+			),
+		);
+	}
+
 	/**
 	 * @ticket 50300
 	 *
@@ -2018,6 +2081,11 @@ class Tests_REST_API extends WP_UnitTestCase {
 				'a,b',
 				array( 'array', 'string' ),
 			),
+			array(
+				'string',
+				'hello',
+				array( 'integer', 'string' ),
+			),
 		);
 	}
 }
diff --git a/tests/phpunit/tests/rest-api/rest-schema-validation.php b/tests/phpunit/tests/rest-api/rest-schema-validation.php
index b31b86bb19..d6d94ae686 100644
--- a/tests/phpunit/tests/rest-api/rest-schema-validation.php
+++ b/tests/phpunit/tests/rest-api/rest-schema-validation.php
@@ -1040,4 +1040,20 @@ class WP_Test_REST_Schema_Validation extends WP_UnitTestCase {
 		$data[1] = array_reverse( $data[1] );
 		$this->assertTrue( rest_validate_value_from_schema( $data, $schema ) );
 	}
+
+	/**
+	 * @ticket 50300
+	 */
+	public function test_string_or_integer() {
+		$schema = array(
+			'type' => array( 'integer', 'string' ),
+		);
+
+		$this->assertTrue( rest_validate_value_from_schema( 'garbage', $schema ) );
+		$this->assertTrue( rest_validate_value_from_schema( 15, $schema ) );
+		$this->assertTrue( rest_validate_value_from_schema( '15', $schema ) );
+		$this->assertTrue( rest_validate_value_from_schema( '15.5', $schema ) );
+		$this->assertWPError( rest_validate_value_from_schema( 15.5, $schema ) );
+	}
+
 }