diff --git a/jedi/evaluate/__init__.py b/jedi/evaluate/__init__.py
index 3feb4491..a231c39f 100644
--- a/jedi/evaluate/__init__.py
+++ b/jedi/evaluate/__init__.py
@@ -174,6 +174,8 @@ class Evaluator(object):
     def _eval_precedence(self, _precedence):
         left = self.process_precedence_element(_precedence.left)
         right = self.process_precedence_element(_precedence.right)
+        if _precedence.operator == ':':
+            return []
         return precedence.calculate(self, left, _precedence.operator, right)
 
     def _eval_statement_element(self, element):
diff --git a/jedi/evaluate/dynamic.py b/jedi/evaluate/dynamic.py
index 43deb068..74c59563 100644
--- a/jedi/evaluate/dynamic.py
+++ b/jedi/evaluate/dynamic.py
@@ -27,7 +27,6 @@ from jedi.evaluate import imports
 # This is something like the sys.path, but only for searching params. It means
 # that this is the order in which Jedi searches params.
 search_param_modules = ['.']
-import sys
 
 
 class ParamListener(object):
diff --git a/jedi/evaluate/iterable.py b/jedi/evaluate/iterable.py
index 0861e953..f180205e 100644
--- a/jedi/evaluate/iterable.py
+++ b/jedi/evaluate/iterable.py
@@ -475,4 +475,4 @@ def create_indexes_or_slices(evaluator, index_array):
             step = None
         return [Slice(evaluator, start, stop, step)]
     else:
-        return evaluator.process_precedence_element(prec) or []
+        return evaluator.process_precedence_element(prec)
diff --git a/jedi/evaluate/precedence.py b/jedi/evaluate/precedence.py
index 7efb69ba..e28d891a 100644
--- a/jedi/evaluate/precedence.py
+++ b/jedi/evaluate/precedence.py
@@ -183,7 +183,8 @@ def calculate(evaluator, left_result, operator, right_result):
         return result
     else:
         if not left_result or not right_result:
-            return left_result + right_result
+            # illegal slices e.g. cause left/right_result to be None
+            return (left_result or []) + (right_result or [])
 
         for left in left_result:
             for right in right_result:
diff --git a/test/completion/precedence.py b/test/completion/precedence.py
index fc42937c..fb80b899 100644
--- a/test/completion/precedence.py
+++ b/test/completion/precedence.py
@@ -98,3 +98,13 @@ for x in [l(0), l(1), l(2), l(3), l(4), l(5), l(6), l(7), l(8), l(9), l(10),
 
 #? str()
 b[1]
+
+
+# -----------------
+# syntax errors
+# -----------------
+
+# strange slice
+z = sorted([1], key = lambda x : x):
+#? int()
+z