2 files changed, 2 insertions, 0 deletions
diff --git a/src/contract_wrappers/exchange_wrapper.ts b/src/contract_wrappers/exchange_wrapper.ts
index 0eb600ff6..b2e48b36e 100644
--- a/src/contract_wrappers/exchange_wrapper.ts
+++ b/src/contract_wrappers/exchange_wrapper.ts
@@ -630,6 +630,7 @@ export class ExchangeWrapper extends ContractWrapper {
                                 callback: EventCallback): Promise<string> {
         assert.doesBelongToStringEnum('eventName', eventName, ExchangeEvents);
         assert.doesConformToSchema('indexFilterValues', indexFilterValues, schemas.indexFilterValuesSchema);
+        assert.isFunction('callback', callback);
         const exchangeContractAddress = await this.getContractAddressAsync();
         const subscriptionToken = this._subscribe(
             exchangeContractAddress, eventName, indexFilterValues, artifacts.ExchangeArtifact.abi, callback,
diff --git a/src/contract_wrappers/token_wrapper.ts b/src/contract_wrappers/token_wrapper.ts
index 05852742a..ee49e8539 100644
--- a/src/contract_wrappers/token_wrapper.ts
+++ b/src/contract_wrappers/token_wrapper.ts
@@ -258,6 +258,7 @@ export class TokenWrapper extends ContractWrapper {
         assert.isETHAddressHex('tokenAddress', tokenAddress);
         assert.doesBelongToStringEnum('eventName', eventName, TokenEvents);
         assert.doesConformToSchema('indexFilterValues', indexFilterValues, schemas.indexFilterValuesSchema);
+        assert.isFunction('callback', callback);
         const subscriptionToken = this._subscribe(
             tokenAddress, eventName, indexFilterValues, artifacts.TokenArtifact.abi, callback,
         );