diff --git a/src/language/__tests__/parser-test.js b/src/language/__tests__/parser-test.js
index cdab9972bd..6271792e82 100644
--- a/src/language/__tests__/parser-test.js
+++ b/src/language/__tests__/parser-test.js
@@ -21,16 +21,6 @@ function expectSyntaxError(text: string) {
 }
 
 describe('Parser', () => {
-  it('asserts that a source to parse was provided', () => {
-    // $FlowExpectedError[incompatible-call]
-    expect(() => parse()).to.throw('Must provide Source. Received: undefined.');
-  });
-
-  it('asserts that an invalid source to parse was provided', () => {
-    // $FlowExpectedError[incompatible-call]
-    expect(() => parse({})).to.throw('Must provide Source. Received: {}.');
-  });
-
   it('parse provides useful errors', () => {
     let caughtError;
     try {
diff --git a/src/language/__tests__/source-test.js b/src/language/__tests__/source-test.js
index 35b72e4985..31a34aa16d 100644
--- a/src/language/__tests__/source-test.js
+++ b/src/language/__tests__/source-test.js
@@ -4,6 +4,20 @@ import { describe, it } from 'mocha';
 import { Source } from '../source';
 
 describe('Source', () => {
+  it('asserts that a body was provided', () => {
+    // $FlowExpectedError[incompatible-call]
+    expect(() => new Source()).to.throw(
+      'Body must be a string. Received: undefined.',
+    );
+  });
+
+  it('asserts that a valid body was provided', () => {
+    // $FlowExpectedError[incompatible-call]
+    expect(() => new Source({})).to.throw(
+      'Body must be a string. Received: {}.',
+    );
+  });
+
   it('can be Object.toStringified', () => {
     const source = new Source('');
 
diff --git a/src/language/parser.js b/src/language/parser.js
index 5fb83ddbb4..5d7b16186d 100644
--- a/src/language/parser.js
+++ b/src/language/parser.js
@@ -1,6 +1,3 @@
-import inspect from '../jsutils/inspect';
-import devAssert from '../jsutils/devAssert';
-
 import type { GraphQLError } from '../error/GraphQLError';
 import { syntaxError } from '../error/syntaxError';
 
@@ -53,8 +50,8 @@ import type {
 } from './ast';
 import { Kind } from './kinds';
 import { Location } from './ast';
-import { Source } from './source';
 import { TokenKind } from './tokenKind';
+import { Source, isSource } from './source';
 import { DirectiveLocation } from './directiveLocation';
 import { Lexer, isPunctuatorTokenKind } from './lexer';
 
@@ -178,11 +175,7 @@ export class Parser {
   _lexer: Lexer;
 
   constructor(source: string | Source, options?: ParseOptions) {
-    const sourceObj = typeof source === 'string' ? new Source(source) : source;
-    devAssert(
-      sourceObj instanceof Source,
-      `Must provide Source. Received: ${inspect(sourceObj)}.`,
-    );
+    const sourceObj = isSource(source) ? source : new Source(source);
 
     this._lexer = new Lexer(sourceObj);
     this._options = options;
diff --git a/src/language/source.d.ts b/src/language/source.d.ts
index 433bf00e5d..a7df7cbb6b 100644
--- a/src/language/source.d.ts
+++ b/src/language/source.d.ts
@@ -16,3 +16,10 @@ export class Source {
   locationOffset: Location;
   constructor(body: string, name?: string, locationOffset?: Location);
 }
+
+/**
+ * Test if the given value is a Source object.
+ *
+ * @internal
+ */
+export function isSource(source: any): source is Source;
diff --git a/src/language/source.js b/src/language/source.js
index 74a823d6fa..05a0fc9c81 100644
--- a/src/language/source.js
+++ b/src/language/source.js
@@ -1,6 +1,8 @@
 import { SYMBOL_TO_STRING_TAG } from '../polyfills/symbols';
 
+import inspect from '../jsutils/inspect';
 import devAssert from '../jsutils/devAssert';
+import instanceOf from '../jsutils/instanceOf';
 
 type Location = {|
   line: number,
@@ -24,6 +26,11 @@ export class Source {
     name: string = 'GraphQL request',
     locationOffset: Location = { line: 1, column: 1 },
   ): void {
+    devAssert(
+      typeof body === 'string',
+      `Body must be a string. Received: ${inspect(body)}.`,
+    );
+
     this.body = body;
     this.name = name;
     this.locationOffset = locationOffset;
@@ -42,3 +49,15 @@ export class Source {
     return 'Source';
   }
 }
+
+/**
+ * Test if the given value is a Source object.
+ *
+ * @internal
+ */
+declare function isSource(source: mixed): boolean %checks(source instanceof
+  Source);
+// eslint-disable-next-line no-redeclare
+export function isSource(source) {
+  return instanceOf(source, Source);
+}
diff --git a/src/utilities/__tests__/stripIgnoredCharacters-test.js b/src/utilities/__tests__/stripIgnoredCharacters-test.js
index 335d193a5a..87ebd5b5e2 100644
--- a/src/utilities/__tests__/stripIgnoredCharacters-test.js
+++ b/src/utilities/__tests__/stripIgnoredCharacters-test.js
@@ -6,9 +6,9 @@ import inspectStr from '../../__testUtils__/inspectStr';
 
 import invariant from '../../jsutils/invariant';
 
+import { Lexer } from '../../language/lexer';
 import { parse } from '../../language/parser';
 import { Source } from '../../language/source';
-import { Lexer } from '../../language/lexer';
 
 import { stripIgnoredCharacters } from '../stripIgnoredCharacters';
 
@@ -98,20 +98,6 @@ function expectStripped(docString: string) {
 }
 
 describe('stripIgnoredCharacters', () => {
-  it('asserts that a source was provided', () => {
-    // $FlowExpectedError[incompatible-call]
-    expect(() => stripIgnoredCharacters()).to.throw(
-      'Must provide string or Source. Received: undefined.',
-    );
-  });
-
-  it('asserts that a valid source was provided', () => {
-    // $FlowExpectedError[incompatible-call]
-    expect(() => stripIgnoredCharacters({})).to.throw(
-      'Must provide string or Source. Received: {}.',
-    );
-  });
-
   it('strips ignored characters from GraphQL query document', () => {
     const query = dedent`
       query SomeQuery($foo: String!, $bar: String) {
@@ -130,6 +116,10 @@ describe('stripIgnoredCharacters', () => {
     );
   });
 
+  it('accepts Source object', () => {
+    expect(stripIgnoredCharacters(new Source('{ a }'))).to.equal('{a}');
+  });
+
   it('strips ignored characters from GraphQL SDL document', () => {
     const sdl = dedent`
       """
diff --git a/src/utilities/stripIgnoredCharacters.js b/src/utilities/stripIgnoredCharacters.js
index 15505a4c8e..f4f53aa042 100644
--- a/src/utilities/stripIgnoredCharacters.js
+++ b/src/utilities/stripIgnoredCharacters.js
@@ -1,6 +1,4 @@
-import inspect from '../jsutils/inspect';
-
-import { Source } from '../language/source';
+import { Source, isSource } from '../language/source';
 import { TokenKind } from '../language/tokenKind';
 import { Lexer, isPunctuatorTokenKind } from '../language/lexer';
 import {
@@ -61,12 +59,7 @@ import {
  * """Type description""" type Foo{"""Field description""" bar:String}
  */
 export function stripIgnoredCharacters(source: string | Source): string {
-  const sourceObj = typeof source === 'string' ? new Source(source) : source;
-  if (!(sourceObj instanceof Source)) {
-    throw new TypeError(
-      `Must provide string or Source. Received: ${inspect(sourceObj)}.`,
-    );
-  }
+  const sourceObj = isSource(source) ? source : new Source(source);
 
   const body = sourceObj.body;
   const lexer = new Lexer(sourceObj);