diff --git a/pkg/types/builder.go b/pkg/types/builder.go
index 4ab4ba8e..9ee85681 100644
--- a/pkg/types/builder.go
+++ b/pkg/types/builder.go
@@ -280,6 +280,9 @@ func (g *Builder) buildSchema(sch *schema.Schema, cfg *config.Resource, tfPath [
 				}
 				elemType = paramType
 			}
+		// if unset
+		case types.Nil:
+			elemType = types.Universe.Lookup("string").Type()
 		default:
 			return nil, errors.Errorf("element type of %s should be either schema.Resource or schema.Schema", fieldPath(names))
 		}