diff --git a/jedi/api/classes.py b/jedi/api/classes.py
index 2f09ae72..fce8bc8e 100644
--- a/jedi/api/classes.py
+++ b/jedi/api/classes.py
@@ -369,7 +369,6 @@ class BaseDefinition(object):
         return self.goto(**kwargs)
 
     def _goto(self, only_stubs=False, prefer_stubs=False):
-        assert not (only_stubs and prefer_stubs)
 
         if not self._name.is_value_name:
             return []
diff --git a/jedi/inference/gradual/conversion.py b/jedi/inference/gradual/conversion.py
index cf295a7c..074e29a7 100644
--- a/jedi/inference/gradual/conversion.py
+++ b/jedi/inference/gradual/conversion.py
@@ -130,7 +130,9 @@ def _python_to_stub_names(names, fallback_to_python=False):
 
 
 def convert_names(names, only_stubs=False, prefer_stubs=False, prefer_stub_to_compiled=True):
-    assert not (only_stubs and prefer_stubs)
+    if only_stubs and prefer_stubs:
+        raise ValueError("You cannot use both of only_stubs and prefer_stubs.")
+
     with debug.increase_indent_cm('convert names'):
         if only_stubs or prefer_stubs:
             return _python_to_stub_names(names, fallback_to_python=prefer_stubs)