diff --git a/spacy/tokens/doc.pxd b/spacy/tokens/doc.pxd
index 28b4a4e10..8d4328878 100644
--- a/spacy/tokens/doc.pxd
+++ b/spacy/tokens/doc.pxd
@@ -28,6 +28,8 @@ cdef int token_by_start(const TokenC* tokens, int length, int start_char) except
 cdef int token_by_end(const TokenC* tokens, int length, int end_char) except -2
 
 
+cdef int set_children_from_heads(TokenC* tokens, int length) except -1
+
 cdef class Doc:
     cdef readonly Pool mem
     cdef readonly Vocab vocab