diff --git a/src/util.ts b/src/util.ts
index 926cbfcdf01..81a990ce291 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -1,4 +1,5 @@
 import { addHexPrefix, isValidAddress, bufferToHex } from 'ethereumjs-util';
+import { ethErrors } from 'eth-rpc-errors';
 import { TYPED_MESSAGE_SCHEMA, typedSignatureHash } from 'eth-sig-util';
 import { Transaction, FetchAllOptions } from './transaction/TransactionController';
 import { MessageParams } from './message-manager/MessageManager';
@@ -340,16 +341,20 @@ export function validateTypedSignMessageDataV3(messageData: TypedMessageParams)
 export function validateTokenToWatch(token: Token) {
   const { address, symbol, decimals } = token;
   if (!address || !symbol || typeof decimals === 'undefined') {
-    throw new Error(`Cannot suggest token without address, symbol, and decimals`);
+    throw ethErrors.rpc.invalidParams(`Must specify address, symbol, and decimals.`);
   }
-  if (!(symbol.length < 7)) {
-    throw new Error(`Invalid symbol ${symbol} more than six characters`);
+  if (typeof symbol !== 'string') {
+    throw ethErrors.rpc.invalidParams(`Invalid symbol: not a string.`);
   }
-  if (isNaN(decimals) || decimals > 36 || decimals < 0) {
-    throw new Error(`Invalid decimals ${decimals} must be at least 0, and not over 36`);
+  if (symbol.length > 6) {
+    throw ethErrors.rpc.invalidParams(`Invalid symbol "${symbol}": longer than 6 characters.`);
+  }
+  const numDecimals = parseInt((decimals as unknown) as string, 10);
+  if (isNaN(numDecimals) || numDecimals > 36 || numDecimals < 0) {
+    throw ethErrors.rpc.invalidParams(`Invalid decimals "${decimals}": must be 0 <= 36.`);
   }
   if (!isValidAddress(address)) {
-    throw new Error(`Invalid address ${address}`);
+    throw ethErrors.rpc.invalidParams(`Invalid address "${address}".`);
   }
 }
 
diff --git a/tests/util.test.ts b/tests/util.test.ts
index 6f953f58b3a..dfcc20d3099 100644
--- a/tests/util.test.ts
+++ b/tests/util.test.ts
@@ -371,21 +371,31 @@ describe('util', () => {
           decimals: 0,
           symbol: 'TKN',
         } as any),
-      ).toThrow('Cannot suggest token without address, symbol, and decimals');
+      ).toThrow('Must specify address, symbol, and decimals.');
       expect(() =>
         util.validateTokenToWatch({
           address: '0x1',
           decimals: 0,
           symbol: undefined,
         } as any),
-      ).toThrow('Cannot suggest token without address, symbol, and decimals');
+      ).toThrow('Must specify address, symbol, and decimals.');
       expect(() =>
         util.validateTokenToWatch({
           address: '0x1',
           decimals: undefined,
           symbol: 'TKN',
         } as any),
-      ).toThrow('Cannot suggest token without address, symbol, and decimals');
+      ).toThrow('Must specify address, symbol, and decimals.');
+    });
+
+    it('should throw if symbol is not a string', () => {
+      expect(() =>
+        util.validateTokenToWatch({
+          address: '0xe9f786dfdd9be4d57e830acb52296837765f0e5b',
+          decimals: 0,
+          symbol: { foo: 'bar' },
+        } as any),
+      ).toThrow('Invalid symbol: not a string.');
     });
 
     it('should throw if symbol is more than 6 characters long', () => {
@@ -395,7 +405,7 @@ describe('util', () => {
           decimals: 0,
           symbol: 'TKNTKNTKN',
         } as any),
-      ).toThrow('Invalid symbol TKNTKNTKN more than six characters');
+      ).toThrow('Invalid symbol "TKNTKNTKN": longer than 6 characters.');
     });
 
     it('should throw if invalid decimals', () => {
@@ -412,14 +422,14 @@ describe('util', () => {
           decimals: 38,
           symbol: 'TKN',
         } as any),
-      ).toThrow('Invalid decimals 38 must be at least 0, and not over 36');
+      ).toThrow('Invalid decimals "38": must be 0 <= 36.');
       expect(() =>
         util.validateTokenToWatch({
           address: '0xe9f786dfdd9be4d57e830acb52296837765f0e5b',
           decimals: -1,
           symbol: 'TKN',
         } as any),
-      ).toThrow('Invalid decimals -1 must be at least 0, and not over 36');
+      ).toThrow('Invalid decimals "-1": must be 0 <= 36.');
     });
 
     it('should throw if invalid address', () => {
@@ -429,7 +439,7 @@ describe('util', () => {
           decimals: 0,
           symbol: 'TKN',
         } as any),
-      ).toThrow('Invalid address 0xe9');
+      ).toThrow('Invalid address "0xe9".');
     });
   });