Email fails with the error Failed to convert parameter value from a String to a Int32

To resolve the error, validate that all data types used in the comparison are consistent. If you are certain that the values being leveraged within the AMPScript are the correct and matching data types, it is possible that you are passing a NULL value within your function. 

As an example of the mismatched data type scenario, say you have a Lookup function that compares a variable to an integer data type within a Data Extension:
Lookup("TestLookup","SubscriberKey","LookupFieldNumber",@numberValue)

If you pass a string value in the @numberValue variable, this will fail with the error listed above.

As an example for the NULL value scenario, say you have the same Lookup function that compares a variable to an integer data type within a Data Extension:
Lookup("TestLookup","SubscriberKey","LookupFieldNumber",@numberValue)

If you have passed a NULL value into the @numberValue variable looking up a match to an Integer data type, this will fail with the data type conversion error as well. 

Text fields should be compared to other text fields and integers should be compared to integers.