# Causes of run-time errors for expressions

At run time the application logs errors about invalid expressions to diagnostics under these circumstances:

- The expression is too large. The maximum compiled size for an expression is 8 KB.
- The expression attempts to compare a string to a number.
- The expression attempts to perform an invalid operation on a string. You can only use string tags as operands with the plus (+) arithmetic operator and with relational operators.
- Adding two strings yields a long string. The maximum string length is 82 characters.
- The expression contains an invalid instruction. In this case, the probable cause is file corruption. Recreate the run-time project.
- A tag in the expression is uninitialized, the value is too old (stale), or the value is in error.
- The expression attempts to perform an invalid operation on a floating point number. You cannot perform modulus or bitwise operations on floating point numbers.
- The expression attempts to divide by zero or perform modulus with zero in the denominator.
- The expression attempts to calculate the square root of a negative number.

## See also

About expressions

The Expression editor

String tags in expressions