diff --git a/helper/schema/resource.go b/helper/schema/resource.go
index fbc75bc1c2..8290ba5035 100644
--- a/helper/schema/resource.go
+++ b/helper/schema/resource.go
@@ -112,6 +112,8 @@ type Resource struct {
 	//
 	// For the most part, only computed fields can be customized by this
 	// function.
+	//
+	// This function is only allowed on regular resources (not data sources).
 	CustomizeDiff CustomizeDiffFunc
 
 	// Importer is the ResourceImporter implementation for this resource.
@@ -378,6 +380,11 @@ func (r *Resource) InternalValidate(topSchemaMap schemaMap, writable bool) error
 		if r.Create != nil || r.Update != nil || r.Delete != nil {
 			return fmt.Errorf("must not implement Create, Update or Delete")
 		}
+
+		// CustomizeDiff cannot be defined for read-only resources
+		if r.CustomizeDiff != nil {
+			return fmt.Errorf("cannot implement CustomizeDiff")
+		}
 	}
 
 	tsm := topSchemaMap
diff --git a/helper/schema/resource_test.go b/helper/schema/resource_test.go
index 22bfd1b8cf..60b49619bf 100644
--- a/helper/schema/resource_test.go
+++ b/helper/schema/resource_test.go
@@ -835,6 +835,21 @@ func TestResourceInternalValidate(t *testing.T) {
 			true,
 			false,
 		},
+
+		13: { // non-writable must not define CustomizeDiff
+			&Resource{
+				Read: func(d *ResourceData, meta interface{}) error { return nil },
+				Schema: map[string]*Schema{
+					"goo": &Schema{
+						Type:     TypeInt,
+						Optional: true,
+					},
+				},
+				CustomizeDiff: func(*ResourceDiff, interface{}) error { return nil },
+			},
+			false,
+			true,
+		},
 	}
 
 	for i, tc := range cases {