diff --git a/py/internal.go b/py/internal.go
index bf654631..df0e285c 100644
--- a/py/internal.go
+++ b/py/internal.go
@@ -95,11 +95,12 @@ func Index(a Object) (Int, error) {
 		if err != nil {
 			return 0, err
 		}
+
 		if res, ok := A.(Int); ok {
 			return res, nil
 		}
 
-		return 0, err
+		return 0, ExceptionNewf(TypeError, "__index__ returned non-int: (type %s)", A.Type().Name)
 	}
 
 	return 0, ExceptionNewf(TypeError, "unsupported operand type(s) for index: '%s'", a.Type().Name)
diff --git a/py/range.go b/py/range.go
index fe31497e..a54005c8 100644
--- a/py/range.go
+++ b/py/range.go
@@ -160,6 +160,16 @@ func computeRangeLength(start, stop, step Int) Int {
 	return res
 }
 
+func getIndexWithDefault(i Object, d Int) (Int, error) {
+	if i == None {
+		return d, nil
+	} else if res, err := Index(i); err != nil {
+		return 0, err
+	} else {
+		return res, nil
+	}
+}
+
 func computeNegativeIndex(index, length Int) Int {
 	if index < 0 {
 		index += length
@@ -177,19 +187,19 @@ func computeBoundIndex(index, length Int) Int {
 }
 
 func computeRangeSlice(r *Range, s *Slice) (Object, error) {
-	start, err := Index(s.Start)
+	start, err := getIndexWithDefault(s.Start, 0)
 	if err != nil {
-		start = 0
+		return nil, err
 	}
-	stop, err := Index(s.Stop)
+	stop, err := getIndexWithDefault(s.Stop, r.Length)
 	if err != nil {
-		stop = r.Length
+		return nil, err
 	}
-
-	step, err := Index(s.Step)
+	step, err := getIndexWithDefault(s.Step, 1)
 	if err != nil {
-		step = 1
+		return nil, err
 	}
+
 	if step == 0 {
 		return nil, ExceptionNewf(ValueError, "slice step cannot be zero")
 	}
diff --git a/py/tests/list.py b/py/tests/list.py
index 3e8468b0..c4ffcb00 100644
--- a/py/tests/list.py
+++ b/py/tests/list.py
@@ -111,4 +111,34 @@
 assertRaises(TypeError, lambda: list.sort(s5, key=1))
 assertRaises(TypeError, lambda: list.sort(1))
 
+class Index:
+    def __index__(self):
+        return 1
+
+a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+b = Index()
+assert a[b] == 1
+assert a[b:10] == a[1:10]
+assert a[10:b:-1] == a[10:1:-1]
+
+class NonIntegerIndex:
+    def __index__(self):
+        return 1.1
+
+a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+b = NonIntegerIndex()
+try:
+    a[b]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
+try:
+    a[b:10]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
 doc="finished"
diff --git a/py/tests/range.py b/py/tests/range.py
index 44eca5db..c027b4fe 100644
--- a/py/tests/range.py
+++ b/py/tests/range.py
@@ -116,4 +116,24 @@ def __index__(self):
 assert a[b:10] == a[1:10]
 assert a[10:b:-1] == a[10:1:-1]
 
+class NonIntegerIndex:
+    def __index__(self):
+        return 1.1
+
+a = range(10)
+b = NonIntegerIndex()
+try:
+    a[b]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
+try:
+    a[b:10]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
 doc="finished"
diff --git a/py/tests/string.py b/py/tests/string.py
index 8eb3b691..43487328 100644
--- a/py/tests/string.py
+++ b/py/tests/string.py
@@ -886,4 +886,35 @@ def index(s, i):
 assert uni[7:7:2] == ''
 assert uni[7:7:3] == ''
 
+class Index:
+    def __index__(self):
+        return 1
+
+a = '012345678910'
+b = Index()
+assert a[b] == '1'
+assert a[b:10] == a[1:10]
+assert a[10:b:-1] == a[10:1:-1]
+
+class NonIntegerIndex:
+    def __index__(self):
+        return 1.1
+
+a = '012345678910'
+b = NonIntegerIndex()
+try:
+    a[b]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
+try:
+    a[b:10]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
+
 doc="finished"
diff --git a/py/tests/tuple.py b/py/tests/tuple.py
index 609a687c..8bc6df20 100644
--- a/py/tests/tuple.py
+++ b/py/tests/tuple.py
@@ -20,4 +20,34 @@
 assert a * 0 == ()
 assert a * -1 == ()
 
+class Index:
+    def __index__(self):
+        return 1
+
+a = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+b = Index()
+assert a[b] == 1
+assert a[b:10] == a[1:10]
+assert a[10:b:-1] == a[10:1:-1]
+
+class NonIntegerIndex:
+    def __index__(self):
+        return 1.1
+
+a = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+b = NonIntegerIndex()
+try:
+    a[b]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
+try:
+    a[b:10]
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised"
+
 doc="finished"