diff --git a/src/gratsConfig.ts b/src/gratsConfig.ts
index f4e1dc11..595f7ed5 100644
--- a/src/gratsConfig.ts
+++ b/src/gratsConfig.ts
@@ -61,11 +61,27 @@ const DEFAULT_TYPESCRIPT_HEADER = `/**
  * Do not manually edit. Regenerate by running \`npx grats\`.
  */`;
 
+const VALID_CONFIG_KEYS = new Set([
+  "graphqlSchema",
+  "tsSchema",
+  "nullableByDefault",
+  "strictSemanticNullability",
+  "reportTypeScriptTypeErrors",
+  "schemaHeader",
+  "tsSchemaHeader",
+]);
+
 // TODO: Make this return diagnostics
 export function validateGratsOptions(
   options: ts.ParsedCommandLine,
 ): ParsedCommandLineGrats {
   const gratsOptions = { ...(options.raw?.grats ?? {}) };
+  for (const key of Object.keys(gratsOptions)) {
+    if (!VALID_CONFIG_KEYS.has(key)) {
+      // TODO: Suggest similar?
+      throw new Error(`Grats: Unknown Grats config option \`${key}\``);
+    }
+  }
   if (gratsOptions.nullableByDefault === undefined) {
     gratsOptions.nullableByDefault = true;
   } else if (typeof gratsOptions.nullableByDefault !== "boolean") {
diff --git a/src/printSchema.ts b/src/printSchema.ts
index 46e58666..5562fe45 100644
--- a/src/printSchema.ts
+++ b/src/printSchema.ts
@@ -26,10 +26,7 @@ export function applyTypeScriptHeader(
   config: ConfigOptions,
   code: string,
 ): string {
-  if (config.tsSchemaHeader) {
-    return `${config.tsSchemaHeader}\n${code}`;
-  }
-  return code;
+  return formatHeader(config.tsSchemaHeader, code);
 }
 
 /**
@@ -45,10 +42,7 @@ export function printGratsSDL(
 }
 
 export function applySDLHeader(config: ConfigOptions, sdl: string): string {
-  if (config.schemaHeader) {
-    return `${config.schemaHeader}\n${sdl}`;
-  }
-  return sdl;
+  return formatHeader(config.schemaHeader, sdl);
 }
 
 export function printSDLWithoutMetadata(doc: DocumentNode): string {
@@ -67,3 +61,10 @@ export function printSDLWithoutMetadata(doc: DocumentNode): string {
   });
   return print(trimmed);
 }
+
+function formatHeader(header: string | string[] | null, code: string): string {
+  if (header !== null) {
+    return `${Array.isArray(header) ? header.join("") : header}\n${code}`;
+  }
+  return code;
+}
diff --git a/src/tests/fixtures/configOptions/headerIsArrayWithNumber.invalid.ts b/src/tests/fixtures/configOptions/headerIsArrayWithNumber.invalid.ts
new file mode 100644
index 00000000..29b5ccc1
--- /dev/null
+++ b/src/tests/fixtures/configOptions/headerIsArrayWithNumber.invalid.ts
@@ -0,0 +1,6 @@
+// {"schemaHeader": ["Hello", 1]}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
diff --git a/src/tests/fixtures/configOptions/headerIsArrayWithNumber.invalid.ts.expected b/src/tests/fixtures/configOptions/headerIsArrayWithNumber.invalid.ts.expected
new file mode 100644
index 00000000..96344ff2
--- /dev/null
+++ b/src/tests/fixtures/configOptions/headerIsArrayWithNumber.invalid.ts.expected
@@ -0,0 +1,14 @@
+-----------------
+INPUT
+----------------- 
+// {"schemaHeader": ["Hello", 1]}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
+
+-----------------
+OUTPUT
+-----------------
+Grats: If the Grats config option `schemaHeader` is an array, it must be an array of strings.
\ No newline at end of file
diff --git a/src/tests/fixtures/configOptions/invalidKey.invalid.ts b/src/tests/fixtures/configOptions/invalidKey.invalid.ts
new file mode 100644
index 00000000..9a71483e
--- /dev/null
+++ b/src/tests/fixtures/configOptions/invalidKey.invalid.ts
@@ -0,0 +1,6 @@
+// {"invalidKey": "Oops"}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
diff --git a/src/tests/fixtures/configOptions/invalidKey.invalid.ts.expected b/src/tests/fixtures/configOptions/invalidKey.invalid.ts.expected
new file mode 100644
index 00000000..4aabee9f
--- /dev/null
+++ b/src/tests/fixtures/configOptions/invalidKey.invalid.ts.expected
@@ -0,0 +1,14 @@
+-----------------
+INPUT
+----------------- 
+// {"invalidKey": "Oops"}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
+
+-----------------
+OUTPUT
+-----------------
+Grats: Unknown Grats config option `invalidKey`
\ No newline at end of file
diff --git a/src/tests/fixtures/configOptions/multipleInvalidKeys.invalid.ts b/src/tests/fixtures/configOptions/multipleInvalidKeys.invalid.ts
new file mode 100644
index 00000000..9673fbbf
--- /dev/null
+++ b/src/tests/fixtures/configOptions/multipleInvalidKeys.invalid.ts
@@ -0,0 +1,6 @@
+// {"invalidKey": "Oops", "anotherInvalidKey": "Oops"}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
diff --git a/src/tests/fixtures/configOptions/multipleInvalidKeys.invalid.ts.expected b/src/tests/fixtures/configOptions/multipleInvalidKeys.invalid.ts.expected
new file mode 100644
index 00000000..ca573386
--- /dev/null
+++ b/src/tests/fixtures/configOptions/multipleInvalidKeys.invalid.ts.expected
@@ -0,0 +1,14 @@
+-----------------
+INPUT
+----------------- 
+// {"invalidKey": "Oops", "anotherInvalidKey": "Oops"}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
+
+-----------------
+OUTPUT
+-----------------
+Grats: Unknown Grats config option `invalidKey`
\ No newline at end of file
diff --git a/src/tests/fixtures/configOptions/nonNullableIsNull.invalid.ts b/src/tests/fixtures/configOptions/nonNullableIsNull.invalid.ts
new file mode 100644
index 00000000..64b0aa10
--- /dev/null
+++ b/src/tests/fixtures/configOptions/nonNullableIsNull.invalid.ts
@@ -0,0 +1,6 @@
+// {"tsSchema": null}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
diff --git a/src/tests/fixtures/configOptions/nonNullableIsNull.invalid.ts.expected b/src/tests/fixtures/configOptions/nonNullableIsNull.invalid.ts.expected
new file mode 100644
index 00000000..fef421e5
--- /dev/null
+++ b/src/tests/fixtures/configOptions/nonNullableIsNull.invalid.ts.expected
@@ -0,0 +1,35 @@
+-----------------
+INPUT
+----------------- 
+// {"tsSchema": null}
+/** @gqlType */
+export default class SomeType {
+  /** @gqlField */
+  hello: string;
+}
+
+-----------------
+OUTPUT
+-----------------
+-- SDL --
+type SomeType {
+  hello: String @metadata
+}
+-- TypeScript --
+import { GraphQLSchema, GraphQLObjectType, GraphQLString } from "graphql";
+export function getSchema(): GraphQLSchema {
+    const SomeTypeType: GraphQLObjectType = new GraphQLObjectType({
+        name: "SomeType",
+        fields() {
+            return {
+                hello: {
+                    name: "hello",
+                    type: GraphQLString
+                }
+            };
+        }
+    });
+    return new GraphQLSchema({
+        types: [SomeTypeType]
+    });
+}
diff --git a/src/tests/test.ts b/src/tests/test.ts
index d2c9518c..54d2fe1c 100644
--- a/src/tests/test.ts
+++ b/src/tests/test.ts
@@ -103,14 +103,19 @@ const testDirs = [
         `${fixturesDir}/${fileName}`,
         path.join(__dirname, `../Types.ts`),
       ];
-      const parsedOptions: ParsedCommandLineGrats = validateGratsOptions({
-        options: {},
-        raw: {
-          grats: options,
-        },
-        errors: [],
-        fileNames: files,
-      });
+      let parsedOptions: ParsedCommandLineGrats;
+      try {
+        parsedOptions = validateGratsOptions({
+          options: {},
+          raw: {
+            grats: options,
+          },
+          errors: [],
+          fileNames: files,
+        });
+      } catch (e) {
+        return e.message;
+      }
 
       // https://stackoverflow.com/a/66604532/1263117
       const compilerHost = ts.createCompilerHost(