diff --git a/Lib/test/test_dbm_gnu.py b/Lib/test/test_dbm_gnu.py
--- a/Lib/test/test_dbm_gnu.py
+++ b/Lib/test/test_dbm_gnu.py
@@ -24,6 +24,7 @@
         self.g[b'bytes'] = b'data'
         key_set = set(self.g.keys())
         self.assertEqual(key_set, set([b'a', b'bytes', b'12345678910']))
+        self.assertIn('a', self.g)
         self.assertIn(b'a', self.g)
         self.assertEqual(self.g[b'bytes'], b'data')
         key = self.g.firstkey()
diff --git a/Modules/_gdbmmodule.c b/Modules/_gdbmmodule.c
--- a/Modules/_gdbmmodule.c
+++ b/Modules/_gdbmmodule.c
@@ -290,20 +290,29 @@
 {
     dbmobject *dp = (dbmobject *)self;
     datum key;
+    Py_ssize_t size;
 
     if ((dp)->di_dbm == NULL) {
         PyErr_SetString(DbmError,
                         "GDBM object has already been closed");
         return -1;
     }
-    if (!PyBytes_Check(arg)) {
+    if (PyBytes_Check(arg)) {
+        key.dptr = PyBytes_AS_STRING(arg);
+        key.dsize = PyBytes_GET_SIZE(arg);
+    }
+    else if (PyUnicode_Check(arg)) {
+        key.dptr = PyUnicode_AsUTF8AndSize(arg, &size);
+        key.dsize = size;
+        if (key.dptr == NULL)
+            return -1;
+    }
+    else {
         PyErr_Format(PyExc_TypeError,
-                     "gdbm key must be bytes, not %.100s",
+                     "gdbm key must be bytes or string, not %.100s",
                      arg->ob_type->tp_name);
         return -1;
     }
-    key.dptr = PyBytes_AS_STRING(arg);
-    key.dsize = PyBytes_GET_SIZE(arg);
     return gdbm_exists(dp->di_dbm, key);
 }