diff --git a/src/language/__tests__/lexer-test.js b/src/language/__tests__/lexer-test.js
index 974f003408..cb98d0978f 100644
--- a/src/language/__tests__/lexer-test.js
+++ b/src/language/__tests__/lexer-test.js
@@ -708,6 +708,41 @@ describe('Lexer', () => {
     );
   });
 
+  it('lex does not allow name-start after a number', () => {
+    expectSyntaxError('0xF1', 'Invalid number, expected digit but got: "x".', {
+      line: 1,
+      column: 2,
+    });
+    expectSyntaxError('0b10', 'Invalid number, expected digit but got: "b".', {
+      line: 1,
+      column: 2,
+    });
+    expectSyntaxError(
+      '123abc',
+      'Invalid number, expected digit but got: "a".',
+      { line: 1, column: 4 },
+    );
+    expectSyntaxError('1_234', 'Invalid number, expected digit but got: "_".', {
+      line: 1,
+      column: 2,
+    });
+    expect(() => lexSecond('1ß')).to.throw(
+      'Syntax Error: Cannot parse the unexpected character "\\u00DF".',
+    );
+    expectSyntaxError('1.23f', 'Invalid number, expected digit but got: "f".', {
+      line: 1,
+      column: 5,
+    });
+    expectSyntaxError(
+      '1.234_5',
+      'Invalid number, expected digit but got: "_".',
+      { line: 1, column: 6 },
+    );
+    expect(() => lexSecond('1.2ß')).to.throw(
+      'Syntax Error: Cannot parse the unexpected character "\\u00DF".',
+    );
+  });
+
   it('lexes punctuation', () => {
     expect(lexOne('!')).to.contain({
       kind: TokenKind.BANG,
diff --git a/src/language/lexer.js b/src/language/lexer.js
index f2f3dfa377..577255ef72 100644
--- a/src/language/lexer.js
+++ b/src/language/lexer.js
@@ -447,8 +447,8 @@ function readNumber(source, start, firstCode, line, col, prev): Token {
     code = body.charCodeAt(position);
   }
 
-  // Numbers cannot be followed by . or e
-  if (code === 46 || code === 69 || code === 101) {
+  // Numbers cannot be followed by . or NameStart
+  if (code === 46 || isNameStart(code)) {
     throw syntaxError(
       source,
       position,
@@ -738,3 +738,10 @@ function readName(source, start, line, col, prev): Token {
     body.slice(start, position),
   );
 }
+
+// _ A-Z a-z
+function isNameStart(code): boolean {
+  return (
+    code === 95 || (code >= 65 && code <= 90) || (code >= 97 && code <= 122)
+  );
+}