diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 065b5e7..a02c222 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -626,8 +626,10 @@
     unsigned int from_kind, to_kind;
     void *from_data, *to_data;
 
-    assert(PyUnicode_Check(from));
-    assert(PyUnicode_Check(to));
+    if (!PyUnicode_Check(from) || !PyUnicode_Check(to)) {
+        PyErr_BadInternalCall();
+        return -1;
+    }
 
     if (PyUnicode_READY(from))
         return -1;