diff --git a/lib/handlebars/internal/wrapHelper.js b/lib/handlebars/internal/wrapHelper.js
index 0010eb8c2..29d65b033 100644
--- a/lib/handlebars/internal/wrapHelper.js
+++ b/lib/handlebars/internal/wrapHelper.js
@@ -1,4 +1,9 @@
 export function wrapHelper(helper, transformOptionsFn) {
+  if (typeof helper !== 'function') {
+    // This should not happen, but apparently it does in https://github.com/wycats/handlebars.js/issues/1639
+    // We try to make the wrapper least-invasive by not wrapping it, if the helper is not a function.
+    return helper;
+  }
   let wrapper = function(/* dynamic arguments */) {
     const options = arguments[arguments.length - 1];
     arguments[arguments.length - 1] = transformOptionsFn(options);
diff --git a/spec/regressions.js b/spec/regressions.js
index 3a84dd7da..f6147ad6e 100644
--- a/spec/regressions.js
+++ b/spec/regressions.js
@@ -518,4 +518,13 @@ describe('Regressions', function() {
       sinon.restore();
     });
   });
+
+  describe("GH-1639: TypeError: Cannot read property 'apply' of undefined\" when handlebars version > 4.6.0 (undocumented, deprecated usage)", function() {
+    it('should treat undefined helpers like non-existing helpers', function() {
+      expectTemplate('{{foo}}')
+        .withHelper('foo', undefined)
+        .withInput({ foo: 'bar' })
+        .toCompileTo('bar');
+    });
+  });
 });