diff --git a/helper/schema/schema.go b/helper/schema/schema.go
index 3e16bb3ab..7eb50203d 100644
--- a/helper/schema/schema.go
+++ b/helper/schema/schema.go
@@ -279,12 +279,18 @@ func (m schemaMap) validate(
 	if !ok {
 		if schema.Required {
 			return nil, []error{fmt.Errorf(
-				"%s: required field is not set")}
+				"%s: required field is not set", k)}
 		}
 
 		return nil, nil
 	}
 
+	if !schema.Required && !schema.Optional {
+		// This is a computed-only field
+		return nil, []error{fmt.Errorf(
+			"%s: this field cannot be set", k)}
+	}
+
 	return m.validatePrimitive(k, raw, schema, c)
 }
 
diff --git a/helper/schema/schema_test.go b/helper/schema/schema_test.go
index 66d70bb52..2a8c4b482 100644
--- a/helper/schema/schema_test.go
+++ b/helper/schema/schema_test.go
@@ -582,7 +582,8 @@ func TestSchemaMap_Validate(t *testing.T) {
 		{
 			Schema: map[string]*Schema{
 				"ingress": &Schema{
-					Type: TypeList,
+					Type:     TypeList,
+					Optional: true,
 					Elem: &Resource{
 						Schema: map[string]*Schema{
 							"from": &Schema{
@@ -622,6 +623,22 @@ func TestSchemaMap_Validate(t *testing.T) {
 
 			Err: true,
 		},
+
+		// Computed field set
+		{
+			Schema: map[string]*Schema{
+				"availability_zone": &Schema{
+					Type:     TypeString,
+					Computed: true,
+				},
+			},
+
+			Config: map[string]interface{}{
+				"availability_zone": "bar",
+			},
+
+			Err: true,
+		},
 	}
 
 	for i, tc := range cases {