diff --git a/src/TemplateTag/TemplateTag.js b/src/TemplateTag/TemplateTag.js
index 000541f..4598dcf 100644
--- a/src/TemplateTag/TemplateTag.js
+++ b/src/TemplateTag/TemplateTag.js
@@ -77,7 +77,7 @@ export default class TemplateTag {
       substitutions.shift(),
       resultSoFar,
     );
-    return resultSoFar + substitution + remainingPart;
+    return ''.concat(resultSoFar, substitution, remainingPart);
   }
 
   /**
diff --git a/src/TemplateTag/TemplateTag.test.js b/src/TemplateTag/TemplateTag.test.js
index d06cead..48e6b9d 100644
--- a/src/TemplateTag/TemplateTag.test.js
+++ b/src/TemplateTag/TemplateTag.test.js
@@ -121,3 +121,25 @@ test('supports passing string as a first argument', () => {
   expect(raw).toBe('FOO BAR\n    500');
   expect(onSubstitutionCalls).toBe(0);
 });
+
+test('transforms substitutions to string as per spec', () => {
+  const get = jest
+    .fn()
+    .mockImplementationOnce((target, prop) => {
+      expect(prop).toBe(Symbol.toPrimitive);
+    })
+    .mockImplementationOnce((target, prop) => {
+      expect(prop).toBe('toString');
+    })
+    .mockImplementationOnce((target, prop) => {
+      expect(prop).toBe('valueOf');
+      return () => 42;
+    });
+
+  const val = new Proxy({}, { get });
+  const tag = new TemplateTag();
+  const result = tag`foo ${val} bar`;
+
+  expect(get).toHaveBeenCalledTimes(3);
+  expect(result).toBe('foo 42 bar');
+});