diff --git a/gluon/sqlhtml.py b/gluon/sqlhtml.py
index 409ad9dc..11206dbd 100644
--- a/gluon/sqlhtml.py
+++ b/gluon/sqlhtml.py
@@ -1535,8 +1535,20 @@ class SQLFORM(FORM):
                 if not value is None:
                     fields[fieldname] = safe_int(value)
             elif field.type.startswith('reference'):
-                if not value is None and isinstance(self.table, Table) and not keyed:
-                    fields[fieldname] = safe_int(value)
+                ## Avoid "constraint violation" exception when you have a
+                ## optional reference field without the dropdown in form. I.e.,
+                ## a field with code to be typed, in a data-entry form.
+                ##
+                ## When your reference field doesn't have IS_EMPTY_OR()
+                ## validator, "value" will come here as a string. So,
+                ## safe_int() will return 0. In this case, insert will raise
+                ## the constraint violation because there's no id=0 in
+                ## referenced table.
+                if isinstance(self.table, Table) and not keyed:
+                    if not value:
+                        fields[fieldname] = None
+                    else:
+                        fields[fieldname] = safe_int(value)
             elif field.type == 'double':
                 if not value is None:
                     fields[fieldname] = safe_float(value)