diff --git a/spacy/tokens/doc.pyx b/spacy/tokens/doc.pyx
index 536b3582b..8a7d12555 100644
--- a/spacy/tokens/doc.pyx
+++ b/spacy/tokens/doc.pyx
@@ -91,9 +91,9 @@ cdef class Doc:
                 raise ValueError("Stepped slices not supported in Span objects."
                                  "Try: list(doc)[start:stop:step] instead.")
             if i.start is None:
-                i.start = 0
+                i = slice(0, i.stop)
             if i.stop is None:
-                i.stop = len(self)
+                i = slice(i.start, len(self))
             return Span(self, i.start, i.stop, label=0)
 
         if i < 0: