diff --git a/spacy/errors.py b/spacy/errors.py
index ebbd314cd..3dab4e1fb 100644
--- a/spacy/errors.py
+++ b/spacy/errors.py
@@ -105,6 +105,10 @@ class Warnings(object):
     W025 = ("'{name}' requires '{attr}' to be assigned, but none of the "
             "previous components in the pipeline declare that they assign it.")
     W026 = ("Unable to set all sentence boundaries from dependency parses.")
+    W027 = ("Found a large training file of {size} bytes. Note that it may "
+            "be more efficient to split your training data into multiple "
+            "smaller JSON files instead.")
+
 
 
 @add_codes
diff --git a/spacy/gold.pyx b/spacy/gold.pyx
index d3316c5d0..0374825dc 100644
--- a/spacy/gold.pyx
+++ b/spacy/gold.pyx
@@ -13,7 +13,7 @@ import srsly
 
 from .syntax import nonproj
 from .tokens import Doc, Span
-from .errors import Errors, AlignmentError
+from .errors import Errors, AlignmentError, user_warning, Warnings
 from .compat import path2str, basestring_
 from . import util
 
@@ -537,12 +537,16 @@ def _json_iterate(loc):
     loc = util.ensure_path(loc)
     with loc.open("rb") as file_:
         py_raw = file_.read()
+    cdef long file_length = len(py_raw)
+    if file_length > 2 ** 30:
+        user_warning(Warnings.W027.format(size=file_length))
+
     raw = <char*>py_raw
     cdef int square_depth = 0
     cdef int curly_depth = 0
     cdef int inside_string = 0
     cdef int escape = 0
-    cdef int start = -1
+    cdef long start = -1
     cdef char c
     cdef char quote = ord('"')
     cdef char backslash = ord("\\")
@@ -550,7 +554,7 @@ def _json_iterate(loc):
     cdef char close_square = ord("]")
     cdef char open_curly = ord("{")
     cdef char close_curly = ord("}")
-    for i in range(len(py_raw)):
+    for i in range(file_length):
         c = raw[i]
         if escape:
             escape = False
diff --git a/spacy/tests/regression/test_issue4674.py b/spacy/tests/regression/test_issue4674.py
index 5f8d1573f..8d0c32eaa 100644
--- a/spacy/tests/regression/test_issue4674.py
+++ b/spacy/tests/regression/test_issue4674.py
@@ -1,11 +1,12 @@
 # coding: utf-8
 from __future__ import unicode_literals
 
+import pytest
 from spacy.kb import KnowledgeBase
 from spacy.util import ensure_path
-
 from spacy.lang.en import English
-from spacy.tests.util import make_tempdir
+
+from ..tests.util import make_tempdir
 
 
 def test_issue4674():
@@ -15,9 +16,12 @@ def test_issue4674():
 
     vector1 = [0.9, 1.1, 1.01]
     vector2 = [1.8, 2.25, 2.01]
-    kb.set_entities(
-        entity_list=["Q1", "Q1"], freq_list=[32, 111], vector_list=[vector1, vector2]
-    )
+    with pytest.warns(UserWarning):
+        kb.set_entities(
+            entity_list=["Q1", "Q1"],
+            freq_list=[32, 111],
+            vector_list=[vector1, vector2],
+        )
 
     assert kb.get_size_entities() == 1