diff --git a/spacy/errors.py b/spacy/errors.py
index 937596d5d..bfdf71652 100644
--- a/spacy/errors.py
+++ b/spacy/errors.py
@@ -408,6 +408,7 @@ class Errors(object):
     E143 = ("Labels for component '{name}' not initialized. Did you forget to call add_label()?")
     E144 = ("Could not find parameter `{param}` when building the entity linker model.")
     E145 = ("Error reading `{param}` from input file.")
+    E146 = ("Could not access `{path}`.")
 
 
 @add_codes
diff --git a/spacy/kb.pyx b/spacy/kb.pyx
index a300ed34a..28e762653 100644
--- a/spacy/kb.pyx
+++ b/spacy/kb.pyx
@@ -372,7 +372,8 @@ cdef class Writer:
             loc = bytes(loc)
         cdef bytes bytes_loc = loc.encode('utf8') if type(loc) == unicode else loc
         self._fp = fopen(<char*>bytes_loc, 'wb')
-        assert self._fp != NULL, "Could not access %s" % loc
+        if not self._fp:
+            raise IOError(Errors.E146.format(path=loc))
         fseek(self._fp, 0, 0)
 
     def close(self):