diff --git a/src/type/__tests__/definition-test.js b/src/type/__tests__/definition-test.js
index ec27aaf94a..86489c0942 100644
--- a/src/type/__tests__/definition-test.js
+++ b/src/type/__tests__/definition-test.js
@@ -730,7 +730,9 @@ describe('Type System: Interface types must be resolvable', () => {
           resolveType: {},
           fields: { f: { type: GraphQLString } },
         }),
-    ).to.throw('AnotherInterface must provide "resolveType" as a function.');
+    ).to.throw(
+      'AnotherInterface must provide "resolveType" as a function, but got: {}.',
+    );
   });
 });
 
@@ -782,7 +784,9 @@ describe('Type System: Union types must be resolvable', () => {
           types: [ObjectWithIsTypeOf],
         }),
       ),
-    ).to.throw('SomeUnion must provide "resolveType" as a function.');
+    ).to.throw(
+      'SomeUnion must provide "resolveType" as a function, but got: {}.',
+    );
   });
 });
 
@@ -905,7 +909,9 @@ describe('Type System: Object types must be assertable', () => {
           fields: { f: { type: GraphQLString } },
         }),
       );
-    }).to.throw('AnotherObject must provide "isTypeOf" as a function.');
+    }).to.throw(
+      'AnotherObject must provide "isTypeOf" as a function, but got: {}.',
+    );
   });
 });
 
diff --git a/src/type/definition.js b/src/type/definition.js
index d588a565d1..8507c387d6 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -651,12 +651,11 @@ export class GraphQLObjectType {
     this._fields = defineFieldMap.bind(undefined, config);
     this._interfaces = defineInterfaces.bind(undefined, config);
     invariant(typeof config.name === 'string', 'Must provide name.');
-    if (config.isTypeOf) {
-      invariant(
-        typeof config.isTypeOf === 'function',
-        `${this.name} must provide "isTypeOf" as a function.`,
-      );
-    }
+    invariant(
+      config.isTypeOf == null || typeof config.isTypeOf === 'function',
+      `${this.name} must provide "isTypeOf" as a function, ` +
+        `but got: ${inspect(config.isTypeOf)}.`,
+    );
   }
 
   getFields(): GraphQLFieldMap<*, *> {
@@ -724,7 +723,7 @@ function defineFieldMap<TSource, TContext>(
       name: fieldName,
     };
     invariant(
-      isValidResolver(field.resolve),
+      field.resolve == null || typeof field.resolve === 'function',
       `${config.name}.${fieldName} field resolver must be a function if ` +
         `provided, but got: ${inspect(field.resolve)}.`,
     );
@@ -757,11 +756,6 @@ function isPlainObj(obj) {
   return obj && typeof obj === 'object' && !Array.isArray(obj);
 }
 
-// If a resolver is defined, it must be a function.
-function isValidResolver(resolver: mixed): boolean {
-  return resolver == null || typeof resolver === 'function';
-}
-
 export type GraphQLObjectTypeConfig<TSource, TContext> = {|
   name: string,
   interfaces?: Thunk<?Array<GraphQLInterfaceType>>,
@@ -903,12 +897,11 @@ export class GraphQLInterfaceType {
     this.resolveType = config.resolveType;
     this._fields = defineFieldMap.bind(undefined, config);
     invariant(typeof config.name === 'string', 'Must provide name.');
-    if (config.resolveType) {
-      invariant(
-        typeof config.resolveType === 'function',
-        `${this.name} must provide "resolveType" as a function.`,
-      );
-    }
+    invariant(
+      config.resolveType == null || typeof config.resolveType === 'function',
+      `${this.name} must provide "resolveType" as a function, ` +
+        `but got: ${inspect(config.resolveType)}.`,
+    );
   }
 
   getFields(): GraphQLFieldMap<*, *> {
@@ -981,12 +974,11 @@ export class GraphQLUnionType {
     this.resolveType = config.resolveType;
     this._types = defineTypes.bind(undefined, config);
     invariant(typeof config.name === 'string', 'Must provide name.');
-    if (config.resolveType) {
-      invariant(
-        typeof config.resolveType === 'function',
-        `${this.name} must provide "resolveType" as a function.`,
-      );
-    }
+    invariant(
+      config.resolveType == null || typeof config.resolveType === 'function',
+      `${this.name} must provide "resolveType" as a function, ` +
+        `but got: ${inspect(config.resolveType)}.`,
+    );
   }
 
   getTypes(): Array<GraphQLObjectType> {