diff --git a/src/language/markdown-language.js b/src/language/markdown-language.js
index 12cc528b..0e414a96 100644
--- a/src/language/markdown-language.js
+++ b/src/language/markdown-language.js
@@ -185,7 +185,7 @@ export class MarkdownLanguage {
 			!validFrontmatterOptions.has(frontmatterOption)
 		) {
 			throw new Error(
-				`Invalid language option value \`${frontmatterOption}\` for frontmatter. Expected one of \`false\`, \`"yaml"\`, \`"toml"\`, or \`"json"\`.`,
+				`Invalid language option value \`${String(frontmatterOption)}\` for frontmatter. Expected one of \`false\`, \`"yaml"\`, \`"toml"\`, or \`"json"\`.`,
 			);
 		}
 
@@ -194,7 +194,7 @@ export class MarkdownLanguage {
 
 		if (mathOption !== undefined && typeof mathOption !== "boolean") {
 			throw new Error(
-				`Invalid language option value \`${mathOption}\` for math. Expected a boolean.`,
+				`Invalid language option value \`${String(mathOption)}\` for math. Expected a boolean.`,
 			);
 		}
 	}
diff --git a/tests/language/markdown-language.test.js b/tests/language/markdown-language.test.js
index b9c5c4cf..9b8654fa 100644
--- a/tests/language/markdown-language.test.js
+++ b/tests/language/markdown-language.test.js
@@ -58,6 +58,17 @@ describe("MarkdownLanguage", () => {
 						'Invalid language option value `123` for frontmatter. Expected one of `false`, `"yaml"`, `"toml"`, or `"json"`.',
 				},
 			);
+			assert.throws(
+				() => {
+					language.validateLanguageOptions({
+						frontmatter: Symbol("frontmatter"),
+					});
+				},
+				{
+					message:
+						'Invalid language option value `Symbol(frontmatter)` for frontmatter. Expected one of `false`, `"yaml"`, `"toml"`, or `"json"`.',
+				},
+			);
 		});
 
 		it("should not throw an error when `frontmatter` has a correct value in commonmark mode", () => {
@@ -116,6 +127,17 @@ describe("MarkdownLanguage", () => {
 						"Invalid language option value `123` for math. Expected a boolean.",
 				},
 			);
+			assert.throws(
+				() => {
+					language.validateLanguageOptions({
+						math: Symbol("math"),
+					});
+				},
+				{
+					message:
+						"Invalid language option value `Symbol(math)` for math. Expected a boolean.",
+				},
+			);
 		});
 
 		it("should not throw an error when `math` has a correct value in commonmark mode", () => {