diff --git a/package.json b/package.json
index 2e3c7aa8be2..f7d7bec0833 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "graphql-tools",
-  "version": "4.0.4",
+  "version": "5.0.0-rc.1",
   "description": "Useful tools to create and manipulate GraphQL schemas.",
   "main": "dist/index.js",
   "typings": "dist/index.d.ts",
diff --git a/src/stitching/defaultMergedResolver.ts b/src/stitching/defaultMergedResolver.ts
index cbf754a03a8..6e74611d8b9 100644
--- a/src/stitching/defaultMergedResolver.ts
+++ b/src/stitching/defaultMergedResolver.ts
@@ -15,7 +15,11 @@ const defaultMergedResolver: GraphQLFieldResolver<any, any> = (parent, args, con
   const errorResult = getErrorsFromParent(parent, responseKey);
 
   if (errorResult.kind === 'OWN') {
-    throw locatedError(new Error(errorResult.error.message), info.fieldNodes, responsePathAsArray(info.path));
+    throw locatedError(
+      errorResult.error.originalError || new Error(errorResult.error.message),
+      info.fieldNodes,
+      responsePathAsArray(info.path),
+    );
   }
 
   let result = parent[responseKey];
diff --git a/src/stitching/errors.ts b/src/stitching/errors.ts
index 94ba6851a25..289d8929c73 100644
--- a/src/stitching/errors.ts
+++ b/src/stitching/errors.ts
@@ -2,7 +2,6 @@ import {
   GraphQLResolveInfo,
   responsePathAsArray,
   ExecutionResult,
-  GraphQLFormattedError,
   GraphQLError,
 } from 'graphql';
 import { locatedError } from 'graphql/error';
@@ -18,7 +17,7 @@ if (
   ERROR_SYMBOL = '@@__subSchemaErrors';
 }
 
-export function annotateWithChildrenErrors(object: any, childrenErrors: ReadonlyArray<GraphQLFormattedError>): any {
+export function annotateWithChildrenErrors(object: any, childrenErrors: ReadonlyArray<GraphQLError>): any {
   if (!childrenErrors || childrenErrors.length === 0) {
     // Nothing to see here, move along
     return object;
@@ -33,10 +32,16 @@ export function annotateWithChildrenErrors(object: any, childrenErrors: Readonly
       }
       const index = error.path[1];
       const current = byIndex[index] || [];
+
+      // It's important to keep the `originalError`, to make sure
+      // error stacktrace's and custom `extensions` are preserved from
+      // source schemas, when merged.
       current.push({
         ...error,
-        path: error.path.slice(1)
+        path: error.path.slice(1),
+        originalError: error.originalError,
       });
+
       byIndex[index] = current;
     });
 
@@ -62,10 +67,10 @@ export function getErrorsFromParent(
     }
   | {
       kind: 'CHILDREN';
-      errors?: Array<GraphQLFormattedError>;
+      errors?: Array<GraphQLError>;
     } {
   const errors = (object && object[ERROR_SYMBOL]) || [];
-  const childrenErrors: Array<GraphQLFormattedError> = [];
+  const childrenErrors: Array<GraphQLError> = [];
 
   for (const error of errors) {
     if (!error.path || (error.path.length === 1 && error.path[0] === fieldName)) {
@@ -114,7 +119,7 @@ export function checkResultAndHandleErrors(
 
   let resultObject = result.data[responseKey];
   if (result.errors) {
-    resultObject = annotateWithChildrenErrors(resultObject, result.errors as ReadonlyArray<GraphQLFormattedError>);
+    resultObject = annotateWithChildrenErrors(resultObject, result.errors as ReadonlyArray<GraphQLError>);
   }
   return resultObject;
 }
diff --git a/src/test/testMergeSchemas.ts b/src/test/testMergeSchemas.ts
index 184f57cfca8..7e93104fece 100644
--- a/src/test/testMergeSchemas.ts
+++ b/src/test/testMergeSchemas.ts
@@ -2315,6 +2315,38 @@ fragment BookingFragment on Booking {
           ],
         });
       });
+
+      it(
+        'should preserve custom error extensions from the original schema, ' +
+        'when merging schemas',
+        async () => {
+          const propertyQuery = `
+            query {
+              properties(limit: 1) {
+                error
+              }
+            }
+          `;
+
+          const propertyResult = await graphql(
+            propertySchema,
+            propertyQuery,
+          );
+
+          const mergedResult = await graphql(
+            mergedSchema,
+            propertyQuery,
+          );
+
+          [propertyResult, mergedResult].forEach((result) => {
+            expect(result.errors).to.exist;
+            expect(result.errors.length > 0).to.be.true;
+            const error = result.errors[0];
+            expect(error.extensions).to.exist;
+            expect(error.extensions.code).to.equal('SOME_CUSTOM_CODE');
+          });
+        }
+      );
     });
 
     describe('types in schema extensions', () => {
diff --git a/src/test/testingSchemas.ts b/src/test/testingSchemas.ts
index 79c455d43be..b08bb474b79 100644
--- a/src/test/testingSchemas.ts
+++ b/src/test/testingSchemas.ts
@@ -407,7 +407,11 @@ const propertyResolvers: IResolvers = {
 
   Property: {
     error() {
-      throw new Error('Property.error error');
+      const error = new Error('Property.error error');
+      (error as any).extensions = {
+        code: 'SOME_CUSTOM_CODE',
+      };
+      throw error;
     },
   },
 };