diff --git a/hgext/fsmonitor/pywatchman/bser.c b/hgext/fsmonitor/pywatchman/bser.c
--- a/hgext/fsmonitor/pywatchman/bser.c
+++ b/hgext/fsmonitor/pywatchman/bser.c
@@ -175,7 +175,15 @@
     const char* item_name = NULL;
     PyObject* key = PyTuple_GET_ITEM(obj->keys, i);
 
+    // tuple keys should always be str.
+#if PY_MAJOR_VERSION >= 3
+    item_name = PyUnicode_AsUTF8(key);
+#else
     item_name = PyBytes_AsString(key);
+#endif
+    if (item_name == NULL) {
+      goto bail;
+    }
     if (!strcmp(item_name, namestr)) {
       ret = PySequence_GetItem(obj->values, i);
       goto bail;