diff --git a/helper/schema/schema.go b/helper/schema/schema.go
index 4a2fd1714..07bdae680 100644
--- a/helper/schema/schema.go
+++ b/helper/schema/schema.go
@@ -925,6 +925,13 @@ func (m schemaMap) diffSet(
 	oldStr := strconv.Itoa(oldLen)
 	newStr := strconv.Itoa(newLen)
 
+	// Build a schema for our count
+	countSchema := &Schema{
+		Type:     TypeInt,
+		Computed: schema.Computed,
+		ForceNew: schema.ForceNew,
+	}
+
 	// If the set computed then say that the # is computed
 	if computedSet || schema.Computed && !nSet {
 		// If # already exists, equals 0 and no new set is supplied, there
@@ -941,22 +948,16 @@ func (m schemaMap) diffSet(
 			countStr = ""
 		}
 
-		diff.Attributes[k+".#"] = &terraform.ResourceAttrDiff{
+		diff.Attributes[k+".#"] = countSchema.finalizeDiff(&terraform.ResourceAttrDiff{
 			Old:         countStr,
 			NewComputed: true,
-		}
+		})
 		return nil
 	}
 
 	// If the counts are not the same, then record that diff
 	changed := oldLen != newLen
 	if changed || all {
-		countSchema := &Schema{
-			Type:     TypeInt,
-			Computed: schema.Computed,
-			ForceNew: schema.ForceNew,
-		}
-
 		diff.Attributes[k+".#"] = countSchema.finalizeDiff(&terraform.ResourceAttrDiff{
 			Old: oldStr,
 			New: newStr,
diff --git a/helper/schema/schema_test.go b/helper/schema/schema_test.go
index b07892867..94b6b4dab 100644
--- a/helper/schema/schema_test.go
+++ b/helper/schema/schema_test.go
@@ -2608,6 +2608,49 @@ func TestSchemaMap_Diff(t *testing.T) {
 
 			Err: false,
 		},
+
+		{
+			Name: "Set ForceNew marks count as ForceNew if computed",
+			Schema: map[string]*Schema{
+				"ports": &Schema{
+					Type:     TypeSet,
+					Required: true,
+					ForceNew: true,
+					Elem:     &Schema{Type: TypeInt},
+					Set: func(a interface{}) int {
+						return a.(int)
+					},
+				},
+			},
+
+			State: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"ports.#": "3",
+					"ports.1": "1",
+					"ports.2": "2",
+					"ports.4": "4",
+				},
+			},
+
+			Config: map[string]interface{}{
+				"ports": []interface{}{"${var.foo}", 2, 1},
+			},
+
+			ConfigVariables: map[string]ast.Variable{
+				"var.foo": interfaceToVariableSwallowError(config.UnknownVariableValue),
+			},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"ports.#": &terraform.ResourceAttrDiff{
+						Old:         "3",
+						New:         "",
+						NewComputed: true,
+						RequiresNew: true,
+					},
+				},
+			},
+		},
 	}
 
 	for i, tc := range cases {