diff --git a/Lib/test/test_exceptions.py b/Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py
+++ b/Lib/test/test_exceptions.py
@@ -937,6 +937,10 @@
         self.assertEqual(exc.name, 'somename')
         self.assertEqual(exc.path, 'somepath')
 
+        # Issue #15778: ensure that str(ImportError(msg)) returns a str
+        # even when msg isn't a str:
+        exc = ImportError(b'test')
+        self.assertEqual(str(exc), "b'test'")
 
 
 def test_main():
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -26,6 +26,9 @@
 - Issue #14846: importlib.FileFinder now handles the case where the
   directory being searched is removed after a previous import attempt
 
+- Issue #15778: ensure that str(ImportError(msg)) returns a str
+  even when msg isn't a str
+
 Library
 -------
 
diff --git a/Objects/exceptions.c b/Objects/exceptions.c
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -679,7 +679,7 @@
 static PyObject *
 ImportError_str(PyImportErrorObject *self)
 {
-    if (self->msg) {
+    if (self->msg && PyUnicode_CheckExact(self->msg)) {
         Py_INCREF(self->msg);
         return self->msg;
     }