diff --git a/packages/buidler-core/src/internal/core/params/argumentTypes.ts b/packages/buidler-core/src/internal/core/params/argumentTypes.ts
index 9c28f524a7..a27f7184df 100644
--- a/packages/buidler-core/src/internal/core/params/argumentTypes.ts
+++ b/packages/buidler-core/src/internal/core/params/argumentTypes.ts
@@ -272,9 +272,8 @@ export const json: ArgumentType<any> = {
     }
   },
   /**
-   * Check if argument value is of type "json"
-   * "json" validation succeeds if it is of "object map"-like structure or of "array" structure
-   * ie. this excludes 'null', function, numbers, date, regexp, etc.
+   * Check if argument value is of type "json". We consider everything except
+   * undefined to be json.
    *
    * @param argName {string} argument's name - used for context in case of error.
    * @param value {any} argument's value to validate.
@@ -282,12 +281,7 @@ export const json: ArgumentType<any> = {
    * @throws BDLR301 if value is not of type "json"
    */
   validate: (argName: string, value: any): void => {
-    const valueTypeString = Object.prototype.toString.call(value);
-    const isJsonValue =
-      valueTypeString === "[object Object]" ||
-      valueTypeString === "[object Array]";
-
-    if (!isJsonValue) {
+    if (value === undefined) {
       throw new BuidlerError(ERRORS.ARGUMENTS.INVALID_VALUE_FOR_TYPE, {
         value,
         name: argName,
diff --git a/packages/buidler-core/test/internal/core/params/argumentTypes.ts b/packages/buidler-core/test/internal/core/params/argumentTypes.ts
index 189581e931..6c6a59c33f 100644
--- a/packages/buidler-core/test/internal/core/params/argumentTypes.ts
+++ b/packages/buidler-core/test/internal/core/params/argumentTypes.ts
@@ -292,5 +292,15 @@ describe("argumentTypes", () => {
     it("Should parse a string", () => {
       assert.deepEqual(types.json.parse("j", '"a"'), "a");
     });
+
+    it("Should accept anything except undefined as valid", () => {
+      assert.doesNotThrow(() => types.json.validate!("json", 1));
+      assert.doesNotThrow(() => types.json.validate!("json", "asd"));
+      assert.doesNotThrow(() => types.json.validate!("json", [1]));
+      assert.doesNotThrow(() => types.json.validate!("json", { a: 123 }));
+      assert.doesNotThrow(() => types.json.validate!("json", null));
+
+      assert.throws(() => types.json.validate!("json", undefined));
+    });
   });
 });
diff --git a/packages/buidler-core/test/internal/core/runtime-environment.ts b/packages/buidler-core/test/internal/core/runtime-environment.ts
index b59bfae66e..60b3b1826f 100644
--- a/packages/buidler-core/test/internal/core/runtime-environment.ts
+++ b/packages/buidler-core/test/internal/core/runtime-environment.ts
@@ -106,12 +106,6 @@ describe("Environment", () => {
         "some default",
         types.string
       )
-      .addOptionalParam(
-        "optJsonParam",
-        "an opt 'json' param",
-        JSON.stringify({ a: 1 }),
-        types.json
-      )
       .addOptionalVariadicPositionalParam(
         "variadicOptStrParam",
         "an opt variadic 'str' param",
@@ -223,7 +217,6 @@ describe("Environment", () => {
           optIntParam: { valid: 10, invalid: 1.2 },
           optFloatParam: { valid: 1.2, invalid: NaN },
           optStringParam: { valid: "a string", invalid: 123 },
-          optJsonParam: { valid: { a: 20 }, invalid: 1234 },
           optFileParam: { valid: __filename, invalid: __dirname },
           variadicOptStrParam: { valid: ["a", "b"], invalid: ["a", 1] },
         };