diff --git a/packages/babel-parser/src/tokenizer/state.js b/packages/babel-parser/src/tokenizer/state.js
index b46819164f8d..35ddf50a8cda 100644
--- a/packages/babel-parser/src/tokenizer/state.js
+++ b/packages/babel-parser/src/tokenizer/state.js
@@ -33,7 +33,11 @@ export default class State {
 
   init(options: Options): void {
     this.strict =
-      options.strictMode === false ? false : options.sourceType === "module";
+      options.strictMode === false
+        ? false
+        : options.strictMode === true
+        ? true
+        : options.sourceType === "module";
 
     this.curLine = options.startLine;
     this.startLoc = this.endLoc = this.curPosition();
diff --git a/packages/babel-parser/test/options.js b/packages/babel-parser/test/options.js
new file mode 100644
index 000000000000..6aa494f9db0e
--- /dev/null
+++ b/packages/babel-parser/test/options.js
@@ -0,0 +1,59 @@
+import { parse } from "../lib";
+
+describe("options", () => {
+  describe("strictMode", () => {
+    const CODE = "function f(x, x) {}";
+
+    function expectToSucceed(opts) {
+      expect(parse(CODE, opts).program.body[0]).toMatchObject({
+        type: "FunctionDeclaration",
+        id: { type: "Identifier", name: "f" },
+        generator: false,
+        async: false,
+        params: [
+          { type: "Identifier", name: "x" },
+          { type: "Identifier", name: "x" },
+        ],
+        body: {
+          type: "BlockStatement",
+          body: [],
+          directives: [],
+        },
+      });
+    }
+
+    function expectToFail(opts) {
+      expect(() => parse(CODE, opts)).toThrow(
+        new SyntaxError("Argument name clash. (1:14)"),
+      );
+    }
+
+    describe("sourceType module", () => {
+      it("default parses as strict mode", () => {
+        expectToFail({ sourceType: "module" });
+      });
+
+      it("false parses as sloppy mode", () => {
+        expectToSucceed({ sourceType: "module", strictMode: false });
+      });
+
+      it("true parses as strict mode", () => {
+        expectToFail({ sourceType: "module", strictMode: true });
+      });
+    });
+
+    describe("sourceType script", () => {
+      it("default parses as sloppy mode", () => {
+        expectToSucceed({ sourceType: "script" });
+      });
+
+      it("false parses as sloppy mode", () => {
+        expectToSucceed({ sourceType: "script", strictMode: false });
+      });
+
+      it("true parses as strict mode", () => {
+        expectToFail({ sourceType: "script", strictMode: true });
+      });
+    });
+  });
+});