diff --git a/src/schema.ts b/src/schema.ts
index 7ffaccd91..ff095ba96 100644
--- a/src/schema.ts
+++ b/src/schema.ts
@@ -229,7 +229,14 @@ export default abstract class BaseSchema<
     let base = this;
     let combined = schema.clone();
 
-    const mergedSpec = { ...base.spec, ...combined.spec };
+    const baseSpec = { ...base.spec };
+    // only override with spec values which are not undefine
+    const mergedSpec = Object.keys(combined.spec).reduce((accum, key) => {
+      if (combined.spec[key] !== undefined){
+        accum[key] = combined.spec[key]
+      }
+      return accum;
+    }, baseSpec)
 
     // if (combined.spec.nullable === UNSET)
     //   mergedSpec.nullable = base.spec.nullable;