diff --git a/helper/schema/resource_data.go b/helper/schema/resource_data.go
index 26d607730..b079dd33c 100644
--- a/helper/schema/resource_data.go
+++ b/helper/schema/resource_data.go
@@ -291,6 +291,13 @@ func (d *ResourceData) getSet(
 		return result
 	}
 
+	// If the entire list is computed, then the entire set is
+	// necessarilly computed.
+	if raw.Computed {
+		result.Computed = true
+		return result
+	}
+
 	list := raw.Value.([]interface{})
 	if len(list) == 0 {
 		if len(parts) > 0 {
diff --git a/helper/schema/schema_test.go b/helper/schema/schema_test.go
index 23c283b38..27b685b71 100644
--- a/helper/schema/schema_test.go
+++ b/helper/schema/schema_test.go
@@ -641,6 +641,88 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Err: false,
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"ports": &Schema{
+					Type:     TypeSet,
+					Required: true,
+					Elem:     &Schema{Type: TypeInt},
+					Set: func(a interface{}) int {
+						return a.(int)
+					},
+				},
+			},
+
+			State: nil,
+
+			Config: map[string]interface{}{
+				"ports": []interface{}{"${var.foo}", 1},
+			},
+
+			ConfigVariables: map[string]string{
+				"var.foo": "2" + config.InterpSplitDelim + "5",
+			},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"ports.#": &terraform.ResourceAttrDiff{
+						Old: "0",
+						New: "3",
+					},
+					"ports.0": &terraform.ResourceAttrDiff{
+						Old: "",
+						New: "1",
+					},
+					"ports.1": &terraform.ResourceAttrDiff{
+						Old: "",
+						New: "2",
+					},
+					"ports.2": &terraform.ResourceAttrDiff{
+						Old: "",
+						New: "5",
+					},
+				},
+			},
+
+			Err: false,
+		},
+
+		{
+			Schema: map[string]*Schema{
+				"ports": &Schema{
+					Type:     TypeSet,
+					Required: true,
+					Elem:     &Schema{Type: TypeInt},
+					Set: func(a interface{}) int {
+						return a.(int)
+					},
+				},
+			},
+
+			State: nil,
+
+			Config: map[string]interface{}{
+				"ports": []interface{}{1, "${var.foo}"},
+			},
+
+			ConfigVariables: map[string]string{
+				"var.foo": config.UnknownVariableValue +
+					config.InterpSplitDelim + "5",
+			},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"ports.#": &terraform.ResourceAttrDiff{
+						Old: "0",
+						New: "",
+						NewComputed: true,
+					},
+				},
+			},
+
+			Err: false,
+		},
+
 		{
 			Schema: map[string]*Schema{
 				"ports": &Schema{