diff --git a/lib/rules/property-no-unknown/__tests__/index.js b/lib/rules/property-no-unknown/__tests__/index.js
index 62736625b1..3fbc2b0801 100644
--- a/lib/rules/property-no-unknown/__tests__/index.js
+++ b/lib/rules/property-no-unknown/__tests__/index.js
@@ -285,31 +285,21 @@ testRule({
 });
 
 testRule({
-	skip: true,
 	ruleName,
 	config: [true],
-	customSyntax: 'postcss-css-in-js',
+	customSyntax: 'postcss-styled-syntax',
 
 	accept: [
 		{
-			code: "import styled from 'styled-components';\nexport default styled.div` ${/* sc-prop */ something}: papayawhip; `;",
+			code: "import styled from 'styled-components';\nexport default styled.div`${something}: papayawhip; `;",
 		},
 		{
-			code: "import styled from 'styled-components';\nexport default styled.div` margin-${/* sc-custom 'left' */ rtlSwitch}: 12.5px; `;",
+			only: true,
+			code: "import styled from 'styled-components';\nexport default styled.div`margin-${rtlSwitch}: 12px;`;",
 		},
 		{
 			code: 'const Component = styled.a`\n\t${rule}: 1;\n`',
 		},
-		{
-			code: `
-        import glm from 'glamorous';
-        const Component = glm.a({
-            overflowX: 'auto',
-            overflowWrap: 'auto',
-            flexDirection: 'row',
-        });
-      `,
-		},
 	],
 
 	reject: [
@@ -331,17 +321,6 @@ testRule({
 			line: 2,
 			column: 2,
 		},
-		{
-			code: `
-        import glm from 'glamorous';
-        const Component = glm.a({
-            overflwX: 'auto',
-        });
-      `,
-			message: messages.rejected('overflw-x'),
-			line: 4,
-			column: 12,
-		},
 	],
 });
 
diff --git a/lib/utils/isStandardSyntaxDeclaration.js b/lib/utils/isStandardSyntaxDeclaration.js
index 2f1f8b5787..b4b0c7c7ff 100644
--- a/lib/utils/isStandardSyntaxDeclaration.js
+++ b/lib/utils/isStandardSyntaxDeclaration.js
@@ -7,6 +7,10 @@ const { isRoot, isRule } = require('./typeGuards');
  * @param {string} [lang]
  */
 function isStandardSyntaxLang(lang) {
+	if (!lang) {
+		return true;
+	}
+
 	return lang && (lang === 'css' || lang === 'custom-template' || lang === 'template-literal');
 }