helper/schema: Prevent crash on removal of computed field in CustomizeDiff

helper/schema/schema.go

@@ -427,6 +427,13 @@ func (m schemaMap) Diff(
 		}
 	}
 
+	// Remove any nil diffs just to keep things clean
+	for k, v := range result.Attributes {
+		if v == nil {
+			delete(result.Attributes, k)
+		}
+	}
+
 	// If this is a non-destroy diff, call any custom diff logic that has been
 	// defined.
 	if !result.DestroyTainted && customizeDiff != nil {
@@ -521,13 +528,6 @@ func (m schemaMap) Diff(
 		result = result2
 	}
 
-	// Remove any nil diffs just to keep things clean
-	for k, v := range result.Attributes {
-		if v == nil {
-			delete(result.Attributes, k)
-		}
-	}
-
 	// Go through and detect all of the ComputedWhens now that we've
 	// finished the diff.
 	// TODO

helper/schema/schema_test.go

@@ -3142,6 +3142,53 @@ func TestSchemaMap_Diff(t *testing.T) {
 				"var.foo": interfaceToVariableSwallowError(""),
 			},
 		},
+
+		{
+			Name: "optional, computed, empty string should not crash in CustomizeDiff",
+			Schema: map[string]*Schema{
+				"unrelated_set": {
+					Type:     TypeSet,
+					Optional: true,
+					Elem:     &Schema{Type: TypeString},
+				},
+				"stream_enabled": {
+					Type:     TypeBool,
+					Optional: true,
+				},
+				"stream_view_type": {
+					Type:     TypeString,
+					Optional: true,
+					Computed: true,
+				},
+			},
+
+			State: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"unrelated_set.#":  "0",
+					"stream_enabled":   "true",
+					"stream_view_type": "KEYS_ONLY",
+				},
+			},
+			Config: map[string]interface{}{
+				"stream_enabled":   false,
+				"stream_view_type": "",
+			},
+			CustomizeDiff: func(diff *ResourceDiff, v interface{}) error {
+				v, ok := diff.GetOk("unrelated_set")
+				if ok {
+					return fmt.Errorf("Didn't expect unrelated_set: %#v", v)
+				}
+				return nil
+			},
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"stream_enabled": {
+						Old: "true",
+						New: "false",
+					},
+				},
+			},
+		},
 	}
 
 	for i, tc := range cases {